The usefulness of a lathe is more than doubled if, without adding an intermediate chuck, you can carry a self-centring chuck on a shank fitting the spindle, or use the many tools that are ready-made with shanks of the Morse taper. British Bulldog.

above the original tone is ingenious, but incorrect, as the two points of the stylus were side by side on the same level, and not one above the other, so that whether in recording or reading off the two points acted simultaneously.

I have done as "Old Subscriber" says, used a in. scale, but I am as far off as ever. On my drawing the distance from centre line of spindle to base of Figs. 3 or 4 is, on fin. scale, a little less than fin. Should this not be 3in. exactly, if lathe is a 3in. centre? The large gear in Fig. 4, on fin. scale would come out only 14in. diameter. Now, it seemed to me that all these dimensions were too small, and that the explanation was that the engraver had simply altered the scale to one which suited the space he was to occupy, and that without any regard to whether his scale coincided with the one in the text. If "Old Subscriber" would kindly give dimensions of the large gear-wheel in Fig. 4, it would satisfy all my requirements. suppose I must admit all his replies to my queries are satisfactory. In a very light spindle, such as this is, I have no doubt greater freedom from vibration is obtained by placing the gearing in front; but in a 6in. lathe, such as mine is, I don't think I need be afraid either to drive by large pulley or gearing at the back instead of front, if I found it in any case more convenient to do so. But my experience with a drill spindle is not extensive enough to enable me to say whether the gears where they are shown would not come in the way of some kinds of work. Others with more experience could say. In any case, I prefer driving by band, not teeth, where smoothness of cut is desired, and where slip is of no great consequence. Circular Cutters.-D. G. T.," letter 32095, is, of course, right in his remarks on size of the tracer; but though I was undoubtedly wrong in saying "the shaper plate can only be the same shape as blank when tracer is same diam. as cutter," I was only wrong in the word "only," for it is a matter of indifference whether you make the tracer or the former smaller by double the depth of the teeth to be cut. In the one case, the former will be of the same dimensions as the blank surface; in the other, of the dimensions of the blank at the bottom of a groove or serration. I think the latter is perhaps, on the whole, the more convenient-e.g., if you want to have a deeper tooth groove in one cutter, which is also of a different shape to the one you have been working on, you have to make a new shaper as well as a new tracer; whereas, by my way, you have only to make or fit a new shaper, which will allow the deeper cut. But, indeed, it is nearly the same which you do. The smaller the diameter of the circular tracer, the better it suits some things.

A disc two depths of not less than diameter of cutter, and slipped on beside the cutter, in the case of an angular cutter, serrated only on the face, ought to work well enough, because the disc would bear on the blank on a radius; but in any other shaped cutter it would not do so, and if the groove were to the full depth of the cutter teeth, I fear the disc would prevent the teeth clearing themselves of the shavings, on whichever side it was placed, but for angular cutters I think it ought to do, and will try it.

Thanks for arrangement of slide-rest for cutter making-I have no doubt it would do; but the drill-spindle is hardly so well fitted for the work as the vertical cutter frame to which the arrangement might be perhaps just as easily fitted. But I do not see why the London L. and T. Co. lathe is more suited to it than the ordinary form. Of course the front slide leaves the bed between the heads clear. But perhaps I do not quite understand what is meant by a worm cutter,-I suppose one with spiral teeth. Fredk. Carre. PHONOGRAPHIC RECORDS-FRENCH

TOYS, &c.

[32109.]-THERE are a few matters which I see are connected with my signature in recent numbers that I may briefly answer in the same letter. I cannot answer the question as to the construction of the "Lampadorama," as I had no chance to examine one; but judging by the exterior form, there seem to be two curved mirrors partially surrounding the lamps, and thus condensing the light the picture. Probably a note to "C. Delagrave, te Soufflot No. 15, Paris," would obtain the ded particulars. The suggested explanation by "Caldemone" of the reproduction of sound an octave

PRESSURE AND DENSITY. [32110.]—I QUITE understand, without the aid of his diagram (letter 32089), Mr. Ranyard's proposition that the increase of volume of concentric spherical shells of similar thickness corresponds to the decrease of gravity towards a heavy body at their centre; but I denied, and still deny, that this increase in volume "balances" the decrease of gravity, or that there is any connection whatever between the two things. There are a great many other phenomena which increase or diminish with the square of the distance; but it does not follow that there is any inter-dependence between them. I was content with a simple denial, because I thought that Mr. Ranyard had merely allowed his pen to slip, and that on reconsideration he would find out that he had gone astray; but I now perceive that he has overlooked a fundamental principle in regard to fluid pressure.

That principle is seen in its simplicity in reference to slightly elastic fluids, such as water, the density of which is nearly uniform. In an elementary work now before me the law is set forth in these words:"Liquids exert a pressure which is to be measured by the height of the column, and not by its bulk." And again: "The pressure of a fluid depends not at all on the extent of the fluid in a horizontal direction." The law is, in principle, the same in regard to perfectly elastic fluids, although their density is not uniform. The height to which the mercury rises in a barometertube is not in the slightest degree affected by the enormous spread of the atmosphere. I was, therefore, justified in declaring that density or pressure is quite independent of lateral bulk.

Let Mr. Ranyard turn his diagram upside down, and suppose that its lines represent a segment of the atmosphere tapering to its upper limit, and cut off from communication with the increasing outside strata, or connected only by a pinhole anywhere in the envelope. Would he be surprised to find that the pressure on the area of the larger plane was precisely the same as the pressure, inch for inch, on the smaller area shown on the diagram in its original position? Yet such would be the case, and the inference is obvious that the spread of the strata of the sphere has nothing to do with the pressure or density.

Will Mr. Ranyard allow me to call his attention to my proposition that a lunar atmosphere corresponding with one of equal pressure on the earth, must be six times as dense. In my first letter I fixed the density of a lunar atmosphere capable of restraining evaporation at the freezing temperature, at th of the density of the earth's atmosphere. I will amend the calculation by setting it down at th, or more than an inch of mercury on the earth. I see that he still pleads for the possible existence of a lunar atmosphere, but there one such as this? If there be not, it is obvious that an atmosphere of less thanth of the density of our own atmosphere would indicate that there was, on the moon, no more water or ice to evaporate. February 21.

Ja. Ha.

HINTS FOR EXPERIMENTALISTS. [32111.]-THE other week I proposed the use of a moving slit in photography, combined with means to completely decompose, or partially modify, the rays of light that constitute the image. No high degree of delicacy is needed in the construction and use of the simplest form of such a photographic apparatus, and I hope that additional suggestions on the subject may not be without practical interest. I described negatives that would show the lines of diffraction spectra in bands resulting from successive exposures. It might be well to use a plate considerably longer than the height of the image that is to be impressed, and to give it an intermittent motion in the direction of its greatest length, and simultaneously with, but opposite to, the motion of the slit. The broken bands of varying intensity on this negative could be selectively grouped by the method of varying exposures mentioned in letter

31993. Suppose we transfer certain such lines on to a rubber transfer sheet, with adjustable frame, and next expand the sheet, in one direction only, until the image becomes symmetrical. We shall have, then, a transfer, in which the lines of the diffraction spectra break up the picture into grain suitable for mechanical printing.

In the arrangement with refracting prisms, let us remove what might be called the "discriminating" slit from between analysing prism and sensitive plate, and insert in the plate-carrier a strip of glass bearing a film sensitised precisely as the pictureplate is to be sensitised. Let white light enter the lens, and, after suitable exposure, an impression of the spectrum can be developed on the glass strip in the form of long but excessively narrow bands, indicating by exact gradations the relative degrees of actinic effect upon our sensitive salt of lights of different hues. Give this negative plate or strip a neutral tone, and let it replace the "discrimi nating" slit in the apparatus that has been described. But now between it and the sensitive plate, upon which the picture is to be impressed, we must introduce our synthesising prism, before described, which will receive the various rays of light each more or less subdued in proportion to its relative actinic effect upon the sensitive compound, and will recombine all such rays in one single narrow line extending across the sensitive surface. I do not touch upon mechanical and optical details, and that for the same reason as mentioned in my last. The special object of this arrangement is not to replace, but rather to sup plement, the chemical methods for orthochromatic photography.

If, for the sake of rapidity, the moving screens used in the camera had several parallel slits, at equal distances apart, such screens could be cheaply and accurately multiplied by means of photography. I wish to add that all such mechanisms are equally applicable in the reproduction of images at a distance, by electrical means, by any of the selenium-cell arrangements that have been proposed. I am not familiar with them, but there could be no difficulty in setting, parallel with the slit, a revolving cylinder of equal breadth with the image, and having disposed upon its curved surface, en echelon, a series of selenium cells, of which each one would be exposed, in the course of a complete revolution, at a different position in the line of light. The cylinder would be moved, together with the slit, &c., very slowly across the field of view; but it would revolve upon its axis comparatively rapidly, so that, practically, in every successive position of the slit there would be a complete revolution of the cylinder. Each individual cell would be brought into circuit, during its exposure only, by a commutator similar to those used on dynamos, and the increased current passing through it when subjected to light would depress the corresponding one of a line of thin types in the printing instrument, or otherwise "make its mark". upon paper in a position corresponding to the position in the breadth of the cylinder of such exposed cell. The revolving commutator which would ba used in the receiving instrument must be exactly synchronised with that of the transmitter, and the receiving paper should be moved, with reference to the line of types, at about the same rate as the motion of the slit in the transmitter. If the motion of the paper is exactly regulated, thus by using the analysing arrangements at the transmitting instrument, and by successive operations, various colours could be printed in the picture, corresponding to some extent with the different photographic impressions that would result if a sensitive plate were substituted for the revolving and moving cylinder in the transmitting arrangement.

But let me describe what would, in my opinion, be a simpler method of "phototelegraphy." Take the cylinder of an Edison phonograph, and produce upon it the photographic image in relief. Replace it in the machine, and behind the diaphragm arrange a microphone, circuited with a telephone magnet in a similar machine at the receiving end of the line. Now cover the cylinder of the receiving phonograph with paper, or other material, revolve synchronically with the transmitting cylinder, and let the point in the centre of the receiving telephone diaphragm reproduce the picture by mechanical cutting, or printing, or by electro-chemical means, or else by means of a mirror or otherwise upon a very sensitive photographic surface. Any of the various methods that have been proposed for transmitting sound would be more or less applicable in this scheme for the reproduction of a relief plate. Photographers can produce such a photo relief upon the transmitting cylinder in many different ways; but the idea that is involved suggests to me that ordinary collodion or gelatine films could be readily made upon cylindrical surfaces (to be afterwards stripped, of course), and if such a cylinder was fitted so as to revolve as if rolling on a flat surface, while moving crosswise or up and down in a camera, the resulting negative would be about the same as that upon an ordinary flat plate. A slit would have to be used, of the length of the cylinder, and in a screen placed near to its surface; and the application for which I propose the arrangement is in

panoramic photography, the cylinder being made to roll about a surface having the required curve. Perhaps, also, if such a roller were made a photomechanical printing surface, it might yield a small number of impressions at a more rapid rate than the ordinary flat surface, and might answer to print cheap pictures from a roll of paper.

I think these ideas are both novel and practicable; but wish to say, once for all, that I may offer matter for publication as to novelty of which I could not hold myself responsible; for I have not time at my disposal, except to read the textbooks, and what I believe to be quite original may not be In that case, if any reader would correct me, I would acknowledge that as a favour. A. D.

So.

TELESCOPE FOCUS AND FLATS. [32112.]-MR. CALVER (letter 32083) somewhat misrepresents me; unintentionally, no doubt. I said quantity of light from moon affected the eyenot intensity. The light from a fixed star is far more intense than the light of moon, but is far less in quantity. It is important to remember the difference.

I have been endeavouring to put in a simple way the effect on the performance of a reflecting telescope if the flat were slightly curved; partially to see how much error is admissible. I may add I have been led to this inquiry from finding the

THE TELESCOPE. [32113.]-ON the question "Reflector versus Refractor" it will be interesting to add the experiences of a very juvenile astronomer.

I have made several telescopes for boys at school, and from time to time receive interesting and good reports. I received such a letter on the 20th inst. (and now inclose it to our Editor), and here quote the concluding remarks: "At school there is an excellent 4in. achromatic, but the 5 in. (reflector) exceeds it in every way, especially on Jupiter. Yours truly, RALPH ELLIOT."

Now, what is worthy of drawing attention to is that this little schoolboy speaks from real knowledge and practice: hence the difference between his disinterested and unbiased expressions, and those made sometimes. One point especially appears contradicted-viz., that the reflector is especially suited to the "professional observer."

The whole series of questions involved are, in the experience of this little schoolboy, reflected and answered. I feel sure that less than twelve months ago his father wrote me he was eleven years old. G. Calver.

OPTICAL HINTS.

[32114.]"ORDERIC VITAL," on p. 423 (last vol.) has certainly expressed the thoughts and hopes of many who are trying to make optical instruments, and more especially the lenses for the instrument. Almost every other profession has had an exhaustive treatise, and now why not give amateur lensmakers a treatise? It is not pages of algebraic formule we need so much as it is instructions in the manipulations and testing of lenses; and, I should say, prisms and spectroscopes and plane surfaces. A few chapters on making, testing, and mounting micro-objectives and micro-accessories would be greeted as a boon by many who are anxious to try it. So far as I am able to learn, no very great lensmakers ever learned the trade by serving an apprenticeship. Among the self-made opticians were Herschel, Fraunhofer, and Lassell; and of our own day are Messrs. Common, J. A. Brashear, Spencer, and the immortal Alvan Clark. Who knows that a thorough treatise on the art of lensmaking may not be the stimulus which shall develop an optician who may make some great stride in science? Let us have a treatise on optical instruments which shall be as complete as those on microscopy, gearing, bell-fitting, dynamos, organs, bookbinding, &c. An exchange of ideas will do the best workmen good.

I take a 12in. mirror of 80in. focus as example, with a 2in. flat 10in. within the focus, and suppose it reflecting the rays to a point nearer the large mirror; straight back again, in fact. I make no pretence to mathematical exactness, since it is not needed for my purpose. How much must this mirror be curved to shorten or lengthen the focus in.? In the figure A B is the distance from flat Books which are of any service to the amateur to focus, 10in.; AC is this shortened in. It is are very rare, and none of much practical use. The evident by inspection that CD will be approxi-"E.M." is the place for new ideas, and is the mately 10in., and that the mirror element at medium through which some competent writers margin of flat must be rotated to cause this just may reach many needy amateurs who are struggling enough to deflect the normal E one-half of for supremacy in the grandest and most useful of this amount, that is in. But if it deviates only all sciences. 2000in. in 10in., it will be 20,000in., or about 555yds.,

Б

2

before it cuts the prolongation of the axis of mirror. In other words, the small mirror will cause a shortening of focus of in. if it is part of a curve of 550 yards radius +. Of course, if presented convexly, it will lengthen about the same

amount.

10

R. Gabriel.

Milford Center, Ohio, U.S.A., Feb. 5.

REFRACTION AND DISPERSION OF LIGHT.

[32115.]-WILL you kindly allow me to draw the attention of "F.R.A.S.," "Sigma," or other correspondents of your periodical studying the subject, to the accompanying extracts regarding the refraction and dispersion of light?

In Dr. Tyndall's lectures on "Light," pp. 107-10, is the following:-"The two elements of rapidity of propagation, both of sound and light in any substance whatever, are elasticity and density, the speed increasing with the former and diminishing with the latter. The enormous velocity of light in stellar space is attainable because the ether is at the same time of infinitesimal density and of enormous elasticity. Now the ether surrounds the atoms of all bodies, but is not independent of them. In If we now turn this slightly-curved mirror at an ponderable matter it acts as if its density were angle of 45°, what will be the effect on definition? increased without a proportionate increase of elasThe minor axis will shorten the focus in. The ticity, and this accounts for the diminished velocity major axis, from its position, compresses the curve, of light in refracting bodies. We here reach a so to speak, and has a more powerful action in the point of cardinal importance. In virtue of the proportion of 14 to 10. It will shorten the focus crystalline architecture that we have been considerdin. The difference is in., so that a small ing, the ether in many crystals possesses different mirror of 555 yards focus causes the same defect as if densities, and hence different elasticities, in different the large mirror were in. less focus horizonally directions; the consequence is that in these directions than vertically, so that a good focus becomes im- light is transmitted with different velocities. And possible. The result is as shown in Fig. 2, where as refraction depends wholly upon the change of BA represents the difference of focus of the hori- velocity on entering the refracting medium, being zontal and vertical curve of flat. At A the vertical greatest where the change of velocity is greatest, reflections come to a focus, while the horizontal we have in many crystals two different refractions. reflections are past focus, and at B the horizontal By such crystals a beam of light is divided into reflections are concentrated, while the vertical have two. This effect is called double refraction. not yet reached the focus. The consequence, with a high power, is a line instead of a disc, or, rather, an oval elongated disc in transverse positions, while at intermediate points a cross is formed, with one

I am unwilling to quit this subject before raising it to unmistakable clearness in your minds. The vibrations of light being transversal, the elasticity concerned in the propagation of any ray is the

elasticity at right angles to the direction of propagation. In Iceland spar there is one direction round which the crystalline molecules are symmetrically built. This direction is called the axis of the crystal. In consequence of this symmetry the elasticity is the same in all directions perpendicular to the axis, and hence a ray transmitted along the axis suffers no double refraction. But the elasticity along the axis is greater than the elasticity at right angles to it. Consider, then, a system of waves crossing the crystal in a direction perpendicular to the axis. Two directions of vibration are open to such waves-the ether particles can vibrate parallel to the axis or perpendicular to it. They do both, and hence immediately divide themselves into two systems propagated with different velocities. Double refraction is the necessary consequence."

I have given this extract in full, understanding it to mean that the rate of vibration diminishes with increasing density of the ether, "without a proportional increase of elasticity." But in Dr. Lommel's "Treatise on Light," pp. 244-5, are these words:-"The phenomena of anomalous dispersion renders us strongly disposed to the opinion that neither the refrangibility nor the length of undulation, but the number of vibrations, is to be regarded as the characteristic of a homogeneous ray of light. The number of undulations by which the impression of colour perceived by our eyes is conditioned, does not undergo any alteration in the passage of light from one medium into another. In fact, we observe no change of tint when, for example, the yellow light of sodium passes from air into water. The length of the waves, however, does undergo a change. The wave-length is, it is to be remembered, always to be obtained by dividing the velocity of propagation by the number of vibrations. As the latter remains unchanged, whilst the velocity of propagation in water is only three-fourths of the velocity in air, the wave-length in water can only amount to three-fourths of the wave-length in air. The wave-length of a ray of light in any given substance is consequently obtained by dividing the wavelength in air by the index of refraction of the substance itself."

In a recent letter "Sigma" stated that the ether is incompressible, and in that sense I understand Sir Wm. Thomson's definition of the ether as "an elastic solid, through which the planets move with the greatest ease, being nearly in the same condition, so far as our means of judging are concerned, in our air as in the inter-planetary space."

Being just now in some difficulty amidst this diversity of teaching, I should like the opinions of others, for, as Dr. Tyndall observes, it is a point of cardinal importance in all questions relating to the nature of the ether. O. T. O. H. P.

ORGANIC ELECTRICITY. [32116.]-THE science of what has been called "Electro-Biology " has been recently drifting so lamentably into the hands of the professional mesmeriser, the table-turner, and the spiritualist, that it is necessary for us to try and bring it back to its true position. "Organic electricity" is an extremely important branch of science, if by the term we mean the study of the living animal body in its relations to the electric current, both as an electro-generator and as an electro-meter.

As a preliminary to some few remarks upon electricity as a remedy in disease, which I shall venture to send to our Editor shortly, I wish to draw attention to the most important of those early results of experiments in this science, which I think we do not sufficiently value.

It is rather strange that this branch of electrical science should be the one least studied by electricians, for the primary discovery which led to the knowledge of galvanism was an experiment in organic electricity - Galvani's frog is too well known to need any further reference here.

That the living animal body is an electrogenerator is abundantly proved. The experiments of Aldini in 1803 were the first which led us to this knowledge. Holding the skinned legs of a recentlykilled frog by the toes in his well-moistened hand, and touching the bared spinal cord, which had been left attached to the legs, with his tongue, he found that the muscles of the frog's legs contracted-this means that a current of electricity sets in from the exterior to the interior, or vice-versâ, of the operator's body.

If we consider the two surfaces of the body, we find the whole of the interior mucous membrane, with the exception of the stomach and cæcum, is a large surface bathed with an alkaline fluid, and the exterior-the skin-is another large surface abundantly supplied with an acid fluid. Between these two surfaces there is the animal membrane, the skin, and the mass of the tissues of the body. These are conditions necessary for the existence of a galvanic battery.

The evolution of an electric current in the course of any chemical action is well known, and there are many chemical rearrangements going on in the living body, the slow combustion of carbon and hydrogen in respiration, and the many chemical changes taking place in secretion and assimilation

the question up. It is the electricians who will help us on, not the doctors (they are just now deaf to anything which does not deal with "microbes"), and it is hopeless for "one of the ex-communicate to expect a hearing in the medical journals. I shall send one, or perhaps two, more papers in continuation of the subject." Gerard Smith. Upper Clapton, London, N.E.

NEAR EUSTON.

[32117.]-THE report of the inspector of the Board of Trade upon the above-named accident seems to point to many strange events.

ment against the soundness of my theory. It is possible that the Caribs may have learned the art of constructing the merimba through some European who had worked out the problem. A more likely solution of the difficulty, and one that is rather corroborated by the style of Mr. Captalzi's merimba, is that someone with a good musical ear had empirically discovered how much the resonators improved the quality of the tone, just as musicians found out that the fifth is a harmonious interval thousands of years before the scientific reason for this was dreamt of. I have tested the pitch of some of the notes of the merimba and taken the dimensions of the corresponding resonators, and find that these fit into each other sufficiently well to show clearly that the latter have been made to reinforce the former. I have also come upon a curious converse case where the quality of the note human ear increasing the loudness of a high overtone. This is mentioned in Helmholtz's "Sensations of Tone" (p. 116 of Mr. Ellis's translation), where it favours the tones between e" and g"; and that is stated that the human ear by its own resonance these overtones produce a feeling of pain in sensi. tive ears. The upper partial tones which have nearly this pitch, if any such exist in a particular note, are extremely prominent and affect the ear powerfully; but if a very small glass tube or sphere is applied to the ear, the cutting effect ceases, and these notes become as soft and weak as the rest. As the injurious effect of the resonance of the ear is so far counteracted by the introduction of a resonator tuned to a lower pitch, so conversely could the beneficial effect of the merimba's resonators be no doubt diminished by placing alongside of them resonators tuned to the higher overtones of the rods they lie under.

in the bile.

It is probable that every muscle is in itself a battery, for the exterior of the capillaries in the muscles is acid, from the presence of free lactic and phosphoric acids, and the blood in the vessels is alkaline, the animal membrane of the vessel walls completing the necessary conditions for a battery. The identity of nerve force with electricity is now fairly well established, for if a nerve is connected with a galvanometer at two points, and the part of the brain which acts upon that nerve is irritated, a deflection of the needle occurs.

We have, then, many facts to show that the animal body is a very complex set of batteries, and that the vital actions of life are accompanied with many exhibitions of electric torce. Yet we have only touched the most elementary data of the problem. We have not yet determined whether these currents are the result or the cause of the chemical changes. My own opinion, which I give as a very incomplete and tentative suggestion, is that we must look upon the brain as the central battery, the spinal cord as the commutator and series of switches, which latter are found at the ganglia on the spinal nerveroots, the sensory current (electric) being conveyed to the brain via these ganglia, and the action, or motor current, being sent also by the same route and drawn from the store of energy in the brain.

The word store suggests storage. We ought to find how far the structure of the brain can give us the conditions of a storage battery. The complex arrangement of the white and grey matter, the insulation of the lobes from the various surfaces of membranes, very vascular as some of the latter are, and the fluid contents of the ventricles, which are considerable, all need exact and laborious study from the point of view of the electrician. We have fairly, certainly, fixed the facts that there are many sources of electric currents present in the body, and that these currents are exhibited when irritations or sensory stimulations of the various organs and tissues are made. We have also a considerable amount of proof that from the brain flow outwards electric-action currents, many of them rhythmic and life-long, as the respiratory and circulatory currents. We see also in disease examples of electric or nerve storms, as in epilepsy and other convulsive disorders. These strongly suggest an irregular and tumultuous discharge of electric force due to some as yet unknown defect in the central battery, or in the conducting or insulating apparatus. These things suggest the idea of a storage battery, supplied by many sources through in-going nerves, and discharging by out-going nerves such actioncurrents as are necessary to answer the demands of the organs or tissues at work.

We can overdraw on this store of energy by unnatural stimulation, and if the central battery gets overcharged, we have it in our power to discharge the excess by extra muscular or other exercise.

Railway men will in the first place regard it as a rather strange thing that a goods train left Preston without a bank engine, and "stuck" on the main hill. The driver of the goods train when running lost the whole of his train, and did not know it; the signalman stopped the engine and tender, and allowed the run-away train to dash into it, blocking both roads. At least, all signals should have been put on then, but that was not done. A passenger train rushed into the block; the collision broke the vacuum automatic-brake pipe. That should have pulled the train up by automatic action; but it failed, and the passenger train could not be stopped for over 1,600 yards.

Colonel Rich does not explain this bad failure. Can any readers of the ENGLISH MECHANIC throw any light on it?

A Railway Man.

FROZEN WATER-PIPES.

[32118.]-IN the arrangement described by Mr. Wm. Grimshaw (p. 547) for the prevention of frozen water-pipes, in the issue of last week, no doubt, both in his and his son's houses, the cisterns are placed in a suitable position.

My experience of London suburban residences is that the pipes and the cisterns are placed in very unsuitable positions, especially the hot-water tank, and when the weather is cold, and the kitchen-fire is let out at night, even if the device was adopted, as recommended by Wm. G., the circulation would have stopped before the fire was lighted again in the kitchen, especially during the weather we have recently experienced.

THE MERIMBA.

Sterne.

Alex. Thurburn.

COIL BOILERS. [32120.]-PRESSURE of business has prevented me reading my "E. M." as carefully as usual the past few weeks, consequently the letters on above subject have only just come under my notice. I am very much gratified to hear of "London River's" success with the boiler he has made from my description in "Ours" nearly two years ago, and

I trust he is not the only one who has been successful in any effort they may have made to produce a similar one. He is, however, somewhat in error in describing it as a "Herreshoff," for that boiler has two coils, one inside the other; or, more strictly speaking, one long continuous length of tube bent to form a double coil, one part being wound outside the other, the two forming, of course, a right and left-hand spiral. The feed-water entering the top of inner coil passes down to the bottom, when the greater part would be evaporated; the mingled steam and water then ascends the outer coil, from the top of which it passes away to the engine.

I would advise "London River" to use a higher steam-pressure, for coil boilers are more adapted for high pressures than for comparatively low ones. It must not be forgotten that a given weight of steam at 1001b. pressure occupies practically but one-half the space required for the same at 50lb., and coil boilers have no steam-room to spare. My safetyvalve was loaded to 1201b. per square inch.

No doubt "Solent" (letter 73428) was too hasty in condemning "London River's" kindly reply to "H. B. S.'s" query; but, unfortunately, he is not alone in "Ours" in finding fault with other peoples' practice, simply because it does not fit in with their own theory, and, without due consideration, hasten to throw cold water upon the efforts

[32119.]-THE instrument of this class described by Mr. Caplatzi (letter 31961) is a very ingenious and interesting one. If he has found time to examine the notes yielded by each lug, and those which the resonators connected with them are fitted to reinforce, it would be, no doubt, very acceptable to many of your readers if he would favour them with the results. It is extremely likely that the curved part of the merimba in the possession of the Keith Field Club was used for the purpose which Mr. Caplatzi attributes to the arched third side of his merimba-viz., carrying it about. It was not, however, a mere conjecture on my part that the player goes inside and steadies the instrument by sitting on the curved part. Mr. Porter, magistrate at Stann Creek, Honduras, who brought home the merimba, informed me that it was used in this way. He also is my authority for the statement regarding which Mr. Caplatzi desiderates confirmationviz., that the merimba is tuned by attaching wax weights to the under sides of the rods. Mr. Porter is in a position to know positively as to this, and his view is confirmed by the fact that the King of Siam's harmonicons were tuned in the same way as mentioned in my former letter. The membranes at the lower ends of the cylin-... B. S. & photo. of launch which was fitted I would be pleased to send "London River" and ders are not shown in my sketch, as they are situated at the back of them. They would vibrate with the coil boiler I described, if they will care to in unison with the notes to which the resonators accept it, and like to advertise their addresses in the were tuned; but any increase which this would usual column. cause in the volume of sound would be more than counterbalanced by the diminution of the proper effect of the resonators. It, therefore, seems to me that they are the products of an ignorant fancy, and that the smaller shells on Mr. Caplatzi's resonators are the same. If so, it is a very interesting fact that two instruments having so much in common, and yet differing so materially in some points, should each have a meaningless excrescence, and it would be highly desirable if Mr. Caplatzi could trace out the history of his one, so as to show how far the makers of the two are connected with each other.

I have ventured to send these remarks to the Editor, hoping that some of our readers who are more learned than I am in these matters may take

of others.

John J. Cann.

A CORRECTION. [32121.]-IN my letter, No. 32095, on page 548, line 33, there is a full stop placed after the word mentioned," which completely alters my meaning. It should read as follows:-"keeping the slide carrying the cutter pressed down to its work by the lever before mentioned, in order to keep the guide or tracer in contact with the template." In the letter above mentioned I am made to say that the cutting of the teeth on one side of the blank is to keep the tracer in contact with the template, which D. G. T. is absurd.

GAS GENERATED IN CHARGING ACCUMULATORS. the notes, and that it would be advantageous to [32122.-DISCUSSING systems of electric lighting smother these effects by resonators which would with an insurance agent some time since, he inincrease the volume of the fundamental note, but formed me (with regard to the accumulator system) not that of the overtones. If I am right in sup- that his company always insist on the cells being posing that the membranes are a useless addition, placed in a special house, well ventilated, as two this would strengthen the view that the inventor of cases of fire came under their notice lately through the instrument had no knowledge of the scientific the combustion of gases given off in charging. Not principles involved in it. But this affords no argu-being a practical electrician, he could not give me