JUNE 5, 1891. ENGLISH MECHANIC AND WORLD OF SCIENCE: No. 1367. the resultant of the two forces must obtain, however small either of the components may be, provided combined at right angles, their resultant is equal to the pressure necessary to produce motion. This is equivalent to saying that if, in the case under consideration, motion be produced at all, that motion will be in obedience to both the forces, both along and around the shaft. For similar reasons it follows that the converse of the experiment would be found to hold. If the axle-box be forced along the shaft, the shaft will be turned by any twisting force, however small, supposing, of course, that no resistance external to our experiment has to be overcome. It follows also that two forces, one tending to produce motion on, and the other along the shaft against friction, may produce motion, though either individually may be incompetent to do so; for suppose two pressures, each just short of producing motion independently to act simultaneously, the resultant pressure will be equal to one of them into V2, which, if one force be just short, must be sufficient to produce motion. As a matter of fact each force must be-supposing them equal to each other-equal to the coefficient of friction into the pressure holding the surfaces together into Vi when the force necessary to overcome the inertia of the moving mass is neglected. Again, the problem may be tested by the "principle of work." Suppose a force able to move the surface against friction to act through one unit of distance longitudinally, then through one unit circumferentially, clearly 2f would represent the work that had been expended, and the sliding surface has reached a certain position; but had the surface travelled in a spiral direction, it would have reached the same position by a shorter route, and as the work expended is in proportion to the distance in this latter case, less work would be required-that is, less simultaneously applied forces would have effected the movement. Every mechanic has perhaps experienced this effect. When, say, a rod has been jammed into a tube, we may be unable to directly pull it out or twist it round; but when the pull and twist are exerted simultaneously, the tube may be moved. A. R. Molison. Swansea. THE CAUSE OF RECOIL. [32389.] I THINK that the cause of recoil of a gun or pistol may be explained in a very simple manner. The bullet offers a certain resistance to the explosion, and thus we might say that the gun is fired away from the bullet, the same way the bullet is fired away from the gun, except that the gun offers a much greater resistance than the bullet, therefore there will be a greater force against the bullet. Or, in other words, the force of the explosion acts in every direction, and thus forces the gun backwards. In letter 32380, page says that the bullet leaves the 295, "Oxonian" barrel before the recoil takes place. This is evidently not true, for directly the bullet begins to move, the gun will do the same, as the force acts in every direction at the same time. I think, also, that "Oxonion" is mistaken in his explanation of the difference between the firing of the gun in the experiment and when held. When the gun is held, the butt end will be against the body, and thus will not be able to move, so the direction of the recoil is straight along the barrel, thus raising the end of the gun, and consequently making the bullet hit above the bull's-eye; whereas in the case where the gun is suspended, the butt is free to move in the straight line horizontally; thus the whole gun will move in the straight line of its length, and the course of the shot is not altered. No matter how well the sights of a gun are arranged, the aim will not be accurate. As a proof that the inaccuracy of aim is not due to bad sights (although I do not say a bad sight will not make the aim inaccurate), we know that the straight line of sight is above the course of the bullet, and consequently the bullet will strike below the "bullock's" eye, and therefore would arise the necessity to raise the gun. This shows that it must be the recoil that causes the miss, because of which the gun must be lowered. A. Benjamin. snow, but as a very heavy fall of hail, which was COMMUTATOR FOR SIEMENS [32391.]-I SEND herewith a sketch of new form Section. B strength. An E.M.F. of 20 volts passing through Sm. THERAPEUTIC MAGNETISM. disease which resemble our test cases. Therefore All homœopathy has been founded upon experiment with drugs upon the healthy human body. The exact sphere of action of each medicine has changes produced have been put on record; and this thus been mapped out; the symptoms and physical is the ground from which the homoeopath acts. I know that drugs act in disease in a different way way. It is certain that the actions of the large to that in health-very probably in quite the reverse doses used in experiments in health and in the End View experiments in disease of orthodox medicine have a precisely opposite action to that of the attenuated doses of homoeopathy; but it is just this relation of action under different circumstances which forms the ground of the science. I attempted in my letters on therapeutic electricity, when on the experimental part of the matter, to give some account of the sphere of action of the various forms of current upon the normal body, as a foundation for the suggestions for the therapeutic uses of the current. It is true that we see a considerable want of scientific relation between our laws of drug action and their therapeutic uses; but this is just the failure we are learning to conin health to the therapeutic uses, and does so quer; homoeopathy has done more than orthodoxy in this matter, for it argues from the drug action successfully. sleeve. The holes in the insulating material are the commutator. A is a metal washer, sawn in halves; B is an W. R. W. insulating washer. ELECTRICAL. [32392.]-MR. GOODCHILD seems to think that = = = = 3, and the = +r 3E = 6 E Thus doubling the surface has diminished we should have the values: 6 E 3 E = 6 E 9+4 (2). the = 2, Still, after all, much of our knowledge comes from pure empiricism, and here the mere statement of the patient that he or she is better or cured, after some treatment, may be taken as part proof that it was the treatment which cured. We must take care, however, that we do not forget the beneficent vis medicatrix nature. Every disorder of the body is evidenced to our senses by certain groups of symptoms; these are the signs to show us where nature aid these efforts by selecting such agencies as work is going in the struggle for recovery. We are to in the same direction, producing the same symptoms as we do in homoeopathy, and it is in order that we may handle our weapons with intelligence that we Upper Clapton, May 30th. seek to ascertain the exact sphere of the action of each medicinal agent on the normal human body. [32395.]-I AM Sorry to be unable to agree with on the question of therapeutic magSigma netism. "Sigma" attributes the effects on the so great an authority in electrical matters as human body to imagination only. Therein I (1). cannot agree, with him, for some years ago, suffering from nervous and general debility as the result of a severe shock, and desiring to try the effect of a magnetic belt without expending so much money as was then charged for such appliances, I made one to fit round my waist. Wearing it a short time, I soon discovered that it was much too strong. As to its action on my nervous system there could be no doubt. Since then, having cut up the belt into a number of pieces, I have at intervals used the parts of it, one or more at a time, applying them to various parts of the body-principally to (1). the spine between my shoulders, and to the pit of the stomach. I have occasionally hung the pieces of belt over the bead of my bed; invariably I have (2). found effects to follow, sometimes far pleasant. General verdict arrived at was that 8 powerful agent, but that magnetism is to the best advantage could not be acquired by exsufficient experience to enable me to use it perimenting on my own suffering "vile corpus advertising “bunkum " of electricity or magnetism without much inconvenience. One fact, however, being life is not true; and that magnetism, at any have, I believe, arrived at-and that is, that the rate, does not communicate anything to the human body-(I do not say as much about electricity, for I have not tried currents); but I believe magnetism acts on the nerves by attracting to those near which the magnets are applied, the blood, which Thus doubling the surface has increased the = THE BRITISH WEATHER CHART. [32390.]-YOUR correspondent, "A Fellow of the Royal Astronomical Society," has usually a "free run." Like the parson, he "bangs the innards out C = of the Bible," and no one dare rise up and stop him. He has "banged the innards" out of Jenkins' weather chart, and has lost his head in the process, forgetting that his little mansion and garden do not strength. If r 9, the result would be the same in each comprise all England and the Channel Islands. My own experience has been that in January and case. Hence by altering the arrangement and size February the forecasts, both of barometer and of plates, we obtain favourable or unfavourable thermometer, were fairly correct, and they were results according to the ratio of R to r. strictly and unpleasantly correct as regards the fogs in February, as I know to my sorrow, being in London at the time. During March he was partly [32393.]-AXIS" perpetrates the mistake I have at sea, but the snow foretold on April 1st, invisible to the writer of the criticism, was deep in Cheshire; before mentioned-viz., assuming that an ampère and that foretold on the 31st April appeared, not as is a unit of quantity whereas it signifies current OHM'S LAW. Sm. I strengthens and vitalises them, in the same way as A. L. THE WIMSHURST ALTERNATING double slides and plates, does not weigh more than selected as the point at which the focus should be [32396.]-I HAVE been experimenting with a It would be out of place to stop here to write a treatise on "what to photograph, and how to select it"; but I think all the best masters of the art will bear me out in this, that the selection must be made quietly and deliberately on the lines of definite rules of art, and from Nature itself, and not from an inverted image seen with a blanket tied round one's head. The only two legitimate reasons a photographer has for looking in his camera at all are-first, to assure himself that his view is properly placed on the plate-i.e., that it is all Now the determination of the exact point at which a landscape should be in focus is not so difficult or delicate a matter as the like determination with a portrait, chiefly because of the vastly superior depth of focus of landscape lenses, and I, for one, have quite given up the use of the ground glass at all for this purpose, even in an ordinary camera fitted with that regulation appendage. Instead, I use a little scale I have engraved on the bottom board of the camera, and which I made by observing the actual expansion of the camera for different distances of exact focus. Thus my scale is marked "10, 15, 20, 30, 40, 50, 75, 100 horizon," which indications mean that when the camera is set, say, at 25, or half-way between 20 and 30, any object 25 yards off, right in front of the camera, will be in absolute focus, and so on for the other distances. I do not mark anything between 100 yards and the horizon, because, according to my view, nothing over 100 yards away from the camera ought to be exactly in focus. (The horizon mark is useful for clouds, and all very distant views where there is no foreground.) Practically, I find it is very easy to judge the distance of the object which should be in the best focus, and thus the whole bother and annoyance of the focussing cloth is done away with. One more feature of vital importance is, I think, that the camera shall be quickly brought into action when needed. Those instruments in which compactness is gained by multitudinous unscrewings and complicated foldings are a burden to the flesh in more senses than one, and it is often a real disadvantage, from an art point of view, to have to wait long in putting up a camera. Who does not know how aggravating it is, when the conditions which make a splendid subject are fast melting away, to feel that all the hurry one can make will not get the camera together in time to secure any. thing worth having! Having, then, established these points as a foundation, I will now proceed to show the lines on which I think a camera should be made to secure the objects sought after. in," and upright, and so forth; and, second, to ascertain the proper adjustment of the focus. Both of these matters can be, as I shall show, perfectly If we look at a camera in its essence, without arranged without the use of the focussing screen or reference to old or existing designs, it seems to be the black cloth. But there is another reason for essentially a device for holding a lens at a fixed, or which many a photographer-aye, and good ones, nearly fixed, distance in front of a rigid plate, too!-wants to look in the camera," and that is extraneous light being cut off by a conical or other to find out how much they can get in of a given sub-curtain extending from one to the other. ject. They know what they want: they are sure of their composition, light and shade, and all that; but the question is, "Will it all come in?" "If I get that cottage-roof nicely in on the right, how about the broken-down fence and the cluster of brambles on the left? Must I go further back, or more to the right or left, to get the best actual spot for the camera?" Looking in the camera to determine such points as these is legitimate enough, but what a waste of time and energy! Of course, if you can carry the angle of the lens in your mind's eye, the thing is simple enough; but as this is always difficult, there is a most simple little apparatus which any amateur can make for himself with very little trouble and ingenuity, which will enable the point to be settled at a glance, and ten times more satisfactorily than when one's head is wrapped up in a cloth. Here is a sketch of the arrangement which, though it is so simple, and, I believe, so very well known, is very seldom used (see Fig. 1A). SOME PROPOSED IMPROVEMENTS IN My object in writing this paper is to endeavour to point out some lines on which, I think, the manufacture of cameras - - and specially those designed for use by tourists-might be still further much improved. Perhaps the best thing would be, in the first place, to enumerate a series of conditions which should be counted as essentials. The first and most important is portability. This is perhaps so obvious that it would not be worth mentioning, were it not for the fact that the average cameramaker's ideas on the subject are not very practical. "A neat case 18in. long, 9in. deep, and 7in. or Sin. wide" would be described as a highly portable arrangement" in many a well illustrated catalogue; and if he can add that the camera, packed with six as it is much easier to appreciate exactly what is If you ask a regulation camera-maker how he would propose to attain this essential condition, he will tell you you must have first of all a “baseboard," with an upright" front" to carry the lens and an upright "back" to carry the plate. But why have a baseboard" and a front" at all? The plate on which the photo. is to be taken must be in itself rigid if of glass, and must be held in a rigid frame if of flexible material. Surely the rigidity necessary for this essential feature may be made the foundation for all the required rigidity of the whole camera. Lay a half-plate dark slide on the table, and look at it. Try to bend it or twist it, and I think you will find that, if it is a wellmade affair, it is quite strong enough to carry all the slight strains that need be laid upon it. Now hold a half-plate rectilinear, or landscape lens, at about its focal distance from the slide, and then ask yourself, "Would not half a dozen or so of fine steel knitting-needles, suitably fixed, form an ample support and as rigid as need be to connect these two together?"-and I think if you consider that the lens only weighs a few ounces, and that the breaking strain of the needles would be some tons, it can only be a question of a little comparatively simple engineering to secure a rigid support from such slight means. Here, then, in its essence, is my idea of a camera. The plate, or the frame surrounding the plate, gives the initial rigidity to the whole affair, and the lens is held in front of this, in position, by a light, metal framework, rendered stiff by suitable cross braces in the well-known manner. If this little apparatus were made in thin wood or The obvious objection to this arrangement, that light metal, and fitted with spring hinges (like those "the thing won't stand flat," is of no consequence on Camden whist markers), it would fold up into a whatever. A camera is not wanted to "stand," very flat compass and go easily into the breast or except on its proper stand or support; it is a even waistcoat pocket. No advantage is gained in matter of no real moment that, by our design, walls having it large. The little projection seen below in and gate-posts, and such improvised "stands the sketch, though not necessary, is a great advan- become unsuitable. I wish, however, to leave the tage to serve as a handle. So much for selecting whole question of "stands" to a future paper, the view; now for the actual focussing. This, as a it is one of great importance, and cannot be treated rule, is done in one or other of two ways. Either fully in a parenthesis. what is sought for a general focus," or, with a To return, then, to the camera. I only use the man who knows his business, one particular spot is word "dark slide" in the above paragraphs to enable me to illustrate my meaning; but a little, taken, being wound off on a spool, and another مام FIG. 1A. How, then, you will ask, are you to provide for developed and fixed, could be printed from it at once, as if it were on glass (and with the additional advantage of being able to print it right or left-handed, as desired, this would be quite perfection. In the mean time, if this cannot be had, there are certainly very excellent films on the market not flexible enough to roll up, but light enough to enable one to carry some dozens without the slightest inconvenience. These I should propose to back with a perfectly light-tight paper, and then place them in a stack-one behind another in the camera. If glass plates be adhered to (notwithstanding their weight, they doubtless have very many advantages), I should treat them in the same way, only, of course, in view of portability, much fewer can be taken at a time. There remains in either of these two last cases the mechanical question of getting away the top plate or film after it has been exposed. There are many ways of doing this. With glass plates, perhaps, the best is to have a separate box or bag to attach to the end of the camera with a slit arrangement, so as to shoot the exposed plate into the box, and then having a spring arrangement to push the ENGLISH MECHANIC AND WORLD OF SCIENCE: No. 1367. others up into place. If the films are used, I am To use such a camera as I have now shadowed Description of the drawing: The drawings show a camera for plates 6 by 4. Fig. 1 is a side view, partly in section and partly in elevation. Fig. 2 is an end elevation. camera closed (but without the A frames). Figs. 4, Fig. 3 a plan showing the 5, and 6 are details which will be described hereafter. The same letters apply to the same parts in all the drawings. The camera comprises a frame of hardwood, ABCD, made of the exact size, so that the plates will fit in at the back in a stack, as shown in Fig. 1. In the depth shown in the drawing, I suppose about twelve plates could be put in. The plate to be next exposed rests against a fillet, ff, Fig. 1. It is got into its place by simply shaking the camera with the fillet side downwards. The plate is held in position against the fillet by being pressed against the side of the rolling shutter R, Fig. 1 (this rolling shutter is seen also in Figs. 2 and 6) by a cam projecting from a roller marked C2, Fig. 1, shown also in plan, Fig. 4. When this roller is turned by the projecting part, seen near B, Fig. 4, the cam C, Fig. 1, presses on the end of the plate, driving the other end against the side of the rolling shutter R, Fig. 1. As the plate in this position is slightly above the centre of the rolling shutter R, Fig. 1, and as the tendency of the cam C is to grip the other end of the plate, the latter will be held firmly in position, and will not tend to fall back amongst the stock of plates when the camera is turned up for use. JUNE 5, 1891. partly in Figs. 1 and 2; the end view of one of them the operator cannot blow "dry," a little inter- set to the places corresponding with the distance at from the lens to the plate, must be either a Surrounding the elastic cord L, and extending "bellows" or a bag. I think a bag could be made perhaps to fit in a smaller compass than a bellows, but this is a detail I am not at the moment able to decide. It may be, also, that in Fig. 1, I have not shown space enough between the closed position of the front and the face of the plate for the bag or Each plate (except the last) must, in such an little thicker, or part of its carrying power of spare bellows. If not, then either the camera must be a arrangement, be protected at the back by a light-plates must be sacrificed. tight tacking, consisting either of some kind of pigment, or of orange or other suitable paper, the lens, the grooves for which are shown in Fig. 1. fastened on with gum, and the latter would, II do not figure the lens, nor any of its attachments, A safety-slide must be placed immediately behind think, be preferable. When the exposure is made, as these are matters which may vary indefinitely and it is desired to remove the plate, the camera is but the lens should be so arranged as to lift on and held in such a position that the rolling shutter and off, and not to require a lot of screwing. When the cam roller are vertical or thereabouts. The the safety-slide is closed the lens may be removed, cam is then "eased back" a little from the plate by and as the camera will remain light-tight, the slightly turning the roller C, and the rolling plates will need no further protection. Two or shutter is then turned in the direction shown by the more valves must be made in the bag, or bellows, to arrow, Fig. 1, till the rectangular slit in the shutter enable the air to pass in or out, or the camera will marked S, Figs. 1, 2, and 3, comes in a line with the neither open nor shut. These may be simply tubes plate. The exact position is indicated by a spring bent several times. The camera as shown will give "click" (there is a similar "click" to indicate a distance of about 94in. from the lens to the plate, when the shutter is shut-i.e., when the slit in it is which is ample for a Dallmeyer's rapid rectilinear at right angles with the length of the plate), not lens. Of course, with shorter focus lenses the affair shown in the drawing. The cam roller is now again would be still more compact. turned in the direction shown by the arrow, Fig. 1, and the cam will now drive the plate into the slit in the rolling shutter, whence by shaking the camera in a suitable direction it will readily fall out through the slit S, Figs. 1 and 2. Of course, if this were done in the open, light would enter by the slit S and spoil the other plates; but a brass slide is provided, and by a counterpart, attached to the part E, a bag can be slid on to the end of the camera to receive the plates. The part E is provided with a rolling shutter with a slit in it similar to the one in the camera. Thus, by attaching the bag and opening both rolling shutters, the plate can be shot out into the bag, and when both roller shutters are again closed, the bag can be removed, and the camera again used without any impediment for the next plate. By this means the stock of plates is transferred one by one, after exposure, to the bag. When this bag is empty it will roll up into a space of about 6in. by 1 by 11, and when full it will represent a space about equal to the ordinary double back. The back of the camera (seen in Fig. 1 between A and B) should, I think, be made of vulcanite, as being not liable to warp by heat and damp. When in its place it must be firmly held by suitable fastenings (not shown in the drawing). evident from what I have said hitherto, that so far, It will be the camera might be with advantage made on the roller-slide principle, with the sensitive tissue stretched from one roller to the other, in the wellknown manner of roller slides; but I have explained the above system because I believe it forms a simple and quite satisfactory way of working with plates in a camera of the kind I am describing. The front of the camera is seen packed up in Fig. 3, and in its position during use in Figs. 1 and 2. It will be seen to consist of three parts hinged together, the central part carrying the lens with or without a rising front arrangement as shown, and the two side pieces containing grooved slides, seen a necessary adjunct to this camera; the reason being I have not as yet mentioned the finder, which is that there are so many arrangements possible, that it is difficult to select which would be the most effipurely optical work. I think the best system would cient. Besides, this is a question which turns upon be to have a miniature telescope so arranged that the field of view should comprise the same as that taken by the lens, and so directed that the view in the telescope would agree with that thrown on the plate. Nelson K. Cherrill, May without being yet able to say we have had the COLD WEATHER IN ITALY. spring. From the 21st of March until to-day the [32398.]-"WE have] now arrived at the end of fine days, the pleasant, mild days, may be counted place of the preceding one-slowly, if you will, but on one's fingers. And yet each season takes the with a sure, progressive moment. of years ago, will continue for millions of years to "The precession of the equinoxes, begun millions come. "Two thousand years before Christ the spring era it took place when the sun entered Aries. In equinox was marked by the entrance of the sun into our time the March equinox comes when the sun is the constellation Taurus. At the beginning of our entering into the constellation Aquarius, and so on for four, six, and eight thousand years, when it will enter successively into Capricornus, Sagittarius, and Scorpio. to the constellation Hercules, which occupies the coldest region of the heavens, in addition to the "Now, as these last constellations are the nearest displacement of the seasons, we shall have a gradually progressive, perceptible reduction of the temperature, and perhaps even a cessation of life on our planet. The earth will, however, not arrive at this critical point in less than 18,000 years, epoch st nearest to the cold constellation Hercules. Even which the axis of the earth will be in the position the creation, Daretes for 5552, St. Augustine, St. Ciprian, and St. Jerome for 6001, Herodotus for which Aristarchus predicted for the year 2484 from then we shall still be far from the end of the world, 10,800, Dione for 13,684, Orpheus for 120,000, and Cassander for 1,800,000. who have announced a fixed date for the end of the world are: Bernard of Thuringia for the year 992, and many others for 1000; Arnold of Villeneuve "The prophets and astronomers of the vulgar era for 1335; Stoffler and Regmontano for 1524; John Hilden for 1652, Whiston for 1715-16; Krudner for 1819; Liebistein for 1833; Nostrodamus for 1886. will not, so long as the conditions of animal life shall and, as it appears to me, contains one or two over LAMP CHIMNEYS AND GLASS-WARE. I have had a explanation of the ways of the chimney-glasses of [32399.]-CAN anyone offer any satisfactory as this will afford great facilities for all "snap-ordinary manner, for it had been in use for weeks, shot" work where the instrument has to be held and was protected by a globe, and certainly no It will easily be seen that a camera so light proof and made in Bohemia went" in an extraparaffin lamps? The other night one marked firein the hands; for any exposures under about a drop of water touched it; but suddenly it blew out second. put the point of the alpenstock about an equal chimney in use with a Doty lamp for years, and, as The following arrangement would be a piece and cracked about half its length. The found quite sufficient: Plant the feet wide apart; wick of the lamp was all right. distance in front; grasp the alpenstock firmly with your readers know, that gets hot enough, so that it the left hand, and press it strongly against the left cannot be the mere temperature. If it is due to shoulder, at the same time leaning a little forward. imperfect annealing, how account for the fact that thus make a The arrangement of two legs and an alpenstock the Bohemian chimney lasted well into the sides, and holding the camera in both chimneys," for tumblers and other glass vessels hands (that is, with the first and second fingers and crack even when they are not so much as looked at. "tripod." By pressing the elbows peculiar behaviour of glassware is not confined to thumb of the left hand, and with the whole of the long? This attainable, and, moreover, the advantage is gained right hand), it will be found that great steadiness is that the camera is brought up to nearly the level of the eye, which is at all times an advantage. Saml. Ray, BICYCLE SO WORK. to use a tube which would comfortably reach the Paris, a distance of 363) miles. kilogramme - mètres; P = the force exerted in kilogrammes 14; the speed in mètres per second = 0.8; and the time = hours, it would be interesting to see how this would compare with the above bicycle work of Mr. Mills. Perhaps some of " ours may be in a position to give details and particulars of the journey itself, which I doubt not would be a source of gratification to many readers of the "E.M." Mr. Mills seems to have arrived at Paris in a very exhausted condition-and no wonder; and I can but admire a persistency of will which could endure for so many hours against a seemingly hopeless task of accomplishing the journey in 30 hours, which was the condition required. LATHE MATTERS. M. [32401.]"A. H. S." asks me to explain "Vulcan's method of dividing by evolutes. I am sorry to say I did not quite understand it, and the plan I followed seemed so much simpler that I did not try very hard. Mine is sketched in Vol. XLIV. p. 478. I still like it very much, though for one who is constantly cutting wheels the "click" system is no doubt preferable, and in Vol. XLVI. p. 258, I gave a table of change-wheels and the divisions they produce. In Vol. L. pp. 478 and 501, is described a system of dividing by differences, quoted from the American Machinist. I hope "Vulcan" will see the appeal of "A. H.S." and reappear amongst us. F. A. M. FIGURING LENSES. [32402.]-HAVING read letter 32343, on page 273, by R. Gabriel, on Grinding Lenses, may I ask R. Gabriel if he will kindly give a few hints on the following? How to keep the lens surface true during the polishing? What size and shape to have tools, and if all cross strokes are the best? I find my lenses frequently gain distortion during polishing, and the spherical surfaces become untrue. Poor Glass Grinder. TEA HUMBUG AT £10 12s. 6d. A [32403.]-As an old "English Mechanic," and also a tea-planter of 27 years' experience, I write to warn all readers of "Ours" that the fancy prices lately paid for some teas at home are purely dodges in advertising. The men who pay seventy (70) times its real value for 12lb. of tea are no fools. They know what they are about. There is not a tea-garden in Asam or India that could not do the same trick. It only needs a little collusion amongst some friends and a retailer, and there is no reason why £50 a pound might not be passed off as a bona-fide sale. So I hope no one will be misled. It is simply a trade trick to get notoriety, and it succeeded. Gambling in tea has, I see, lately been started systematically "on 'Change" at home, and as usual, both producer and consumer will be indirectly injured by it. I hope the day is not far distant when all this capitalists' gambling in stocks and produce, especially in food, will be made a penal offence. It rests with the "working classes"-the bulk of the consumers-to insist on reform in these matters, and in the near future. As a planter, I get, say, 4 per cent. to 5 per cent. on the tea I grow; but before it crosses the counter, middle-men have taken their 44 per cent. on it. I daresay other industries are much the same. Also, I would dearly like to see a crushing tax on all "gilded youths," the drones of society; But I must not go too far, or the Editor will put me in the waste-paper basket. RELPIES TO QUERIES. In their answers, Correspondents are respectfully requested to mention, in each instance, the title and number of the query asked. p. 275 has to do with the question under consideration-viz., that, by enlarging or connecting in parallel the plates of primary cells, the internal resistances are reduced in an inverse ratio. That which has any bearing on the subject is based on an assumption-a mathematical fiction, which may be useful looked at from a mathematical' point of view, but most the "E. M." of Feb. 6th, page 513-14 last vol., in current in a primary cell. The primary cell is [73626.]-Church Bells.-A reply appeared in misleading as to the true cause of the increase of certainly is very answer to the question in a previous issue as to the purely and simply an arrangement-a machine, cause of the clapper of a large bell falling on the whereby we convert potential energy into kinetic wrong side when raising it. The answer is from a energy, and looked at from a practical point of view, stock. This I am not prepared to dispute, having to convert latent energy into active energy. Let bell-hanger, that the bell is buried too deeply in the is quite analogous to other machines that are used had experience only as a bell-ringer in England us take a chromic-acid cell containing plates 4in. some years ago, and as such I know that another square. On the practical knowledge we possess, cause is the too hastily swinging the bell at the early we construct the internal arrangement of the cell stage. Now, the remedy is that a strong check should so as to convert the potential energy of the zinc into be kept on it for the first two or three strokes, the ropes the greatest amount of kinetic energy or current being given out very sparingly until the clapper strikes with the least amount of resistance. In overcoming fairly on each side, and then pull unflinchingly and this resistance, there is a certain quantity of energy give out the rope, as this stroke must not be lost on absorbed which appears in the cell in the form of any account, for if the clapper fails after once heat. Finally, we have 25 units of energy or striking on both sides, it is sure to be wrong when ampères set free (on short circuit), and a certain the bell is up, so the best course to pursue would be number of grains of zinc consumed (independent of to let the bell down and commence afresh to save local action). Now if we want double the ampères the trouble of running up to throw it over. I have (50) in the same time, we can construct a cell with never known the above remedy to fail, even with plates double the size which will give us twice the very heavy bells. The reason for delay with this current; but as the resistance increases as the reply will be obvious. square of the current, and we have only doubled the sectional area of the plates, the internal resistance must of necessity be twice-not one-halfwhat it was in the cell one-half the size. Or we can double the current by constructing another cell exactly similar to the one containing plates 4in. square, which will absorb the same energy in overcoming the resistance, consume the same weight of zinc, and set free the same number of ampères. If we join these two cells (containing 4in. square plates) in parallel, we get 25+ 25 = 50 ampères, the sum of the two cells, but the resistance of each cell will remain the same. Hence the total resistance will be twice that of one cell-not one-halfthe zinc consumed twice as much, and the total ampères set free, twice as many. This is what we might naturally expect. Let us take an analogous case. We have a quantity of water which, from its position, possesses potential energy. We construct a turbine to transform the potential into absorbed in overcoming the resistance or friction, kinetic energy. A certain amount of energy is and, finally, energy, say, equal to 25 horse-power is given off by the machine. If we want double the power, we put down another similar turbine, and supply it with water from the same source. Connecting these two turbines to the same shaft 50 horse-power. (in parallel), we get 25+ 25 = Hence the total friction (resistance) will be twice what it was in the single machine, the water conpower set free twice as much. This will be found sumed will be twice as much, and the total horseto be the case in whatever constructive arrangement we may make, to transform potential energy into kinetic energy. HYSTERESIS. Auckland, New Zealand. J. J. MITCHAM. 90 x 80 x 30 x 2 DANOIS. [74034.]-Tempering Tools for Turning Chilled Rolls.-I think that holding the caststeel tools "on the bosh till quite cold" perhaps be one reason why the querist asked the may question. Probably he found that did not answer. 66 Roll-Turner is not the only mechanic who does not know what can be done with cast-iron tools. E R' E. M. [74055.]-Electrical.-I am much obliged to the various gentlemen who have endeavoured to enlighten me as to the above. At the same time, I must say that it is not yet as clear to me as it appears Bottone. They tell me I am right in supposing to be to such authorities as "Sigma" and Mr. that C = That being so, how is it that a few feet of No. 20 c. wire, having practically no resistance, will convey the current from fifteen chromic-acid cells without being heated? The table of wires states that No. 20 wire will only carry 1.5 ampère; yet here is a case of the same wire carrying, or attempting to carry, something like 25 or 30 amps., and not getting even warm. Anyone can see I am still MIXED. [74055.]—Electrical.—I am not aware that I have asked Mr. Bottone for an answer to more than [74254.]-Telephone Lines.-The wire should be as continuous as possible, but there may be as many spans as needed. See pp. 46, 135 of last volume, where information as to erecting telephone lines will be found, with illustrations. "Weird's " queries are not very clearly put, and it is certainly not necessary to solder the wire unless to make a join. W. S. [74263.1-Carbon Transmitters.-All carbon transmitters are covered by the Edison patent. It is the carbon contact which is the essence of the invention—the vital point, so to speak, of the patent. OLIM. [74264.]-Wheatstone Bridge.-If the slide wire consist of 1,000 parts and the slide is near the centre, if it can be adjusted to an accuracy of one division, then the value of x can be determined to an accuracy of 4 per cent. But as soon as the slide is moved towards the end of the scale to balance unequal resistance, the degree of accuracy becomes less. Let a be the standard and ≈ the unknown resistance, then the formula is— 1,000 one question, and that was: "How, by the simple fact of connecting two sets of plates together in parallel, the resistance is reduced one-half, the amount of current being the same in both cases?" I may also inform him that I have no repugnance to perform the experiments he suggests, having at various times performed similar experiments, and also have observed an increase of current as the negative plate was gradually immersed; but I canI might, however, sound a note of warning to not say that I observed a corresponding increase of my English fellow-workmen. I lately overhauled current, except within a limited range. the stock of a Kabulic pedlar, and was quite taken Bottone says: Mr. "As to the measurement of the aback at the marked improvement in the general internal resistance, as described by Hysteresis,' run of histwares-textiles all A1, and beautiful, why no one ever performs it in such a way." I really high art (Eastern) patterns, hardware and was not describing how to measure the internal odds and ends for natives excellent, and was flatter-resistance. The object I had in view was to show ing myself that English manufacturers and work-that the tangent galvanometer, or any other gal- If a=1 and x=1, balance should be obtained with men were at last going ahead, when I looked at the vanometer, tells us nothing of the internal action the slide at 500. If one division out, then labels, and, to my disgust, found that every single of a primary cell, but simply measures the current item he had was-German. Sibsagar, Asam, April 30. S. E. Peal. men LOCUSTS.-The following is the " cutting tioned by "Eos" in his letter on p. 293.- Swarms of locusts still impede traffic at Jhelum. In the Tallaganj district, nearly 400 tons of locusts have been destroyed without appearing to make much difference. Trains are delayed for hours near Rawal Pindi owing to the plague."-May 9th, 1891. or energy set free at the terminals. And if we adopt the method of measuring the internal resistance-which, I presume, Mr. Bottone considers the orthodox plan-that is, by placing resistances in the showing an error of 004 ohm. If a=1 and x=9, circuit, we are not one wit nearer a solution, for the the slide should stand at 100. If one division out, galvanometer only gives indications depending on the current circulating in its coils; the resistance introduced in the circuit simply transforms a certain amount of current into heat. It may be quite possible, as stated by "Sm.," that I have begun at the wrong end; but as yet I have had no proof that I have done so. On the other hand, I have a strong impression that the road he is travelling THE Tower Bridge will, it is stated, be opened for along is somewhat of a circular one. I cannot see affic in the autumn of 1892. |