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the numbers of divisions covered, which will of course depend upon the power that has been employed.
The use of such a scale will correct erroneous impressions, and I trust convince many that the pea is much nearer the mark than the orange.
I am, Sir, respectfully yours, St. John's Wood : Feb. 10, 1866.
NATH. E. GREEN.
TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,—Your correspondent “ C. A.,” in his letter in your February number, propounds a difficulty which almost everyone who dabbles in scientific matters must have encountered, but for which I fear he seeks in vain for a generally applicable remedy. It is astonishing how few persons have any idea of angular measurement. I have frequently caused members of my own family to look at the moon through a telescope magnifying nearly twelve diameters, who have declared that it seemed to them no larger through the glass than it did to their naked eye. I have tried to make them keep both eyes open, so as to bring the two objects one over the other, in order thus to judge of the magnifying power of the telescope, but they have declared that they still see only one of course, the one through the glass. But is “C. A.” himself aware of the great difficulty of estimating distances, or magnitudes, or perspective effects with the eye only; that is, without any known object to compare them with ? Let him, for instance, close one eye, and let some one hold up before him by a string a ball; he will find himself unable to say whether it is a billiard ball at three feet distance or a marble at one. Perhaps at first he would hardly credit it, that a small pea held up at arm's length would eclipse the sun, or that a fourpenny piece would be three times, a shilling four times, and half-a-crown six times the diameter of the sun.
Whenever I have heard an unpractised person speak of having seen a meteor, I have managed to arrive at something like an idea in the following manner:- I have made him hold up some object at arm's length, and look at it with one eye only, closing the other, and then ask him to describe as nearly as he can what proportion the one has borne to the other; whether the tail, for instance, were as long as his thumb, whether the path were as long as half his walking-stick or umbrella; the former would give five or six degrees for the tail, the latter about forty-five for the path: for arm's length is about two feet, the thumb about two inches, and half the walkingstick about eighteen. For brightness, a candle held before his eye at different distances will sometimes answer the purpose. But this closing one and using only one eye is, in respect of magnitude or brightness, very essential and tolerably trustworthy, as it very much does away with any arbitrary or preconceived ideas of magnitude in the observer's mind.
For velocity, the easiest way is to get him, still with one eye closed, to stretch out his arm, and pass his finger across the heavens, or turn himself round at about the pace at which the meteor appeared to travel, from which the enquirer, by completing the circle, may judge of the angular velocity, Then again, when an object is described as right overhead, this must not be accepted as the zenith, but an allowance of some fifteen or perhaps twenty degrees must be made for his unconsciousness of the difficulty of throwing back his head and casting up the eyes sufficiently: much also depends upon whether the observer contracts or expands his eyelids, as the nearly-closed eye is affected by the eyelashes causing a radiation from which the more open eye is comparatively free. If a candle or a gaslight be looked at with the eyes well open to a distance of three or four feet, no unusual effect will be noticed; but if the eye or eyes be slighly closed, two small pencils of rays will be seen to issue from it upwards and downwards in the form of slightly expanding hollow cones, apex to apex. If the head be gently turned horizontally from side to side, the eye still steadily fixed upon the light, the cones will appear to rotate on their axes, the side nearest to the eye turning in the same direction with the head, and being generally the brighter. Again, a light, when viewed from a distance of forty or fifty yards with the eyelids naturally open, will be observed to have eight principal clumps of rays issuing from it, drawing off to points, the vertical and horizontal being bolder than the diagonal. If the head be greatly rolled or rocked, the eyes being fixed steadily on the light, these rays will be seen to rotate or revolve round the light in the same direction with the rolling of the head. I have never seen any reason given for this particular number eight, nor can I offer one. The same effect of vertical rotation or revolution will be observed in the cones of rays above described.
I am, Sir, faithfully yours,
PATH OF A DETONATING FIREBALL.
TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,—The fireball period of the 20th of November has of late years been chiefly distinguished by the appearance of detonating meteors. It appears, moreover, to be characterised as a period of detonating meteors by an observation of a very early date. In Part II. of his Ephemerides, published about the year 1629, Kepler has described the appearance of a fireball of very unusual dimensions. It passed over a great part of the Austrian dominions from east to west on the 17th of November 1623 (N.s.), and was followed by a loud explosion. The date of Kepler's meteor, corrected to the apparent equinox of 1850, would coincide with the present position of the Earth in its orbit on the 21st of November. Meteors of a detonating class appeared in England on the 19th of November 1861, on the 20th of November 1864, and on the 21st of November 1865. The last of these meteors, coinciding with Kepler's meteor in its date, was observed by Mr. Warren De la Rue, near Cranford. It was also seen by Mr. F. C. Penrose at Wimbledon, by Mr. H. Todd at Cambridge, and by Mr. H. Weightman at Oundle, as well as by other observers at distant places, to whom particular acknowledgments are due for their interesting notes of the phenomenon. It has been possible from these accounts to determine, at least approximately, the real altitude, position, and velocity of the meteor.
At Wimbledon, a loud report, like that of a cannon some miles off, was heard by Mr. F. C. Penrose, about 2m. 2os. after the disappearance of the meteor. The report is stated to have been loud and distinct, and to have been heard by more than one person at Wimbledon. Sound, with its ordinary velocity in common air, would take 2m. 5os. to travel 36 miles—the distance, found from these accounts, at which the meteor disappeared from Wimbledon. A closer agreement of the time could hardly be expected from the accounts if the nature of the phenomenon is considered, and particularly the difficulty of fixing the exact position of the apparent path of a meteor by the several observers. There appears, therefore, no reason to doubt that the report heard at Wimbledon was a meteoric detonation, and that the fireball itself was really one of a detonating class.
At Cranford the meteor appeared to have a diameter of 8' or 10' of arc, and was fully three times as bright as Venus is at its brightest. It first appeared surrounded by a halo of light formed in the vapours of the horizon from which it rose. At an altitude of about 40° due East, it disengaged itself from the haze, and presented the appearance of a fireball from a Roman candle, shining with a brilliant blueish light, accompanied by a tail of a reddish colour, from 23° to 3° in length. The outpouring and falling behind of the matter forming the tail was distinctly visible, as it passed with a slow motion and with evident though small pulsations, lower than, and south of, the stars of the constellation Cassiopeia, almost overhead towards the west. At Wimbledon the disappearance was quite sudden, or at any rate the loss of light was exceedingly rapid, and the meteor vanished some 8° North-West of a Lyre. As the distance of Wimbledon from Cranford is little more than ten miles, both of the observations at these places may be referred to a middle point between them—at Richmond-on-Thames-in comparison with their much greater distance from the other points of observation.
At Hawkhurst (distant 46 miles from Richmond in a direct line, 50° E. from S.), the meteor was seen, through clouds, descending obliquely towards the WSW. horizon, from an altitude of 55° due North.
At Danbury (distant 44 miles from Richmond in a direct line, 65° E. from N.), the meteor, when first seen, was about 20° S. from the zenith, and descended towards the western horizon.
At the Observatory, Cambridge (distant from Richmond 55 miles in a direct line, 19° E. from N.), Mr. H. Todd, assistant at the Cambridge Observatory, noted the apparent path of the meteor from B Aquarii, across @ Aquila, to 8 Aquile.
At Oundle (distant from Richmond 72 miles in a direct line, 50 W. from N.), the apparent track of the meteor was likewise referred to the constellations, from a little East of · Ceti, half a degree below that star, and half a degree below & Aquarii, disappearing some distance below 8 Capricorni.
Near St. Albans (distant from Richmond 20 miles in a direct line, 3o W. from N.), the meteor first made its appearance at an altitude of 58° above the SE. horizon, and moved horizontally from East to West.
At Liverpool (N. lat. 53° 24' 39", W. long. 2° 59' 30", distant from Richmond 175 miles in a direct line, 39° W. from N.), the meteor was first visible 250 or 30° above the SE. horizon, and disappeared at the same altitude SSW.
It requires to be noticed here that, in A Plan for Observing Fireballs, printed in the year 1783 by the whilom Astronomer Royal, Dr. Maskelyne, it is directed that altitudes near the horizon, estimated without the use of instruments, should in general be corrected to obtain the real altitude. The correction amounts to half the original altitude, by which the altitude is usually estimated in excess; so that, at an altitude of 23°, for example, persons may think a meteor at 45°. An altitude of not quite 15° is therefore employed in the following table as the supposed apparent altitude of the meteor above the horizon, as observed at Liverpool:
From the real altitudes, and from the apparent directions of the points of first appearance and disappearance of the meteor as seen from Cranford and from Wimbledon, the real position of the path may easily be determined. This is 75 miles in length; from 41 miles in height above the Nore, to 27 miles in height over the town of Henley-upon-Thames.
At Shoeburyness, near the Nore, where the meteor was seen to make its appearance in the zenith, the time of flight was estimated at about 78.; at Wimbledon, şs.; at Cranford, 109.; and, at Oundle, 48. were recorded as the time occupied by the meteor in its transit. The mean of these quantities, or 6 s., gives for the velocity of the fireball 11 miles per second. The real direction of the flight was from due East, descending at an inclination of 10° to the horizon. Referring this point to the constellations, the real direction of the meteor's flight was from the neighbourhood of the star 1 Tauri.
It deserves to be remarked as a point of the highest interest, and requiring particular attention with a view of determining further points of radiation of the November meteors, that the constellation Taurus was a very prominent region of radiation during the meteoric shower from Leo, eight days before the detonating meteor, on the night of the 12-13th of November last. The velocities of two of these shooting-stars from Taurus, determined on that date, were 18 miles and 23 miles per second-an unusually small velocity (when the velocity of shooting-stars is considered), like the velocity of the detonating meteor on the 21st of November last.
I am, Sir, your obedient servant, Collingwood, Hawkhurst:
ALEX. S. HERSCHEL. Jan. 20, 1866.
OBSERVATIONS AT MENTONE.
TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,- I shall be glad if you will accord me space in the Register for the few following remarks.
Between the 21st and 28th ult. the atmosphere here was unusually steady. There was scarcely any limit to the magnifying power which the stars bore.
Antares. On the mornings of the 22nd, 27th, and 28th, the Companion was distinctly visible with my Cooke's 43-inch refractor before and up to sunrise, free from the light of the large star. The green colour of the Companion was in fine contrast with the brilliant red of the large star.
a Ophiuci 4, 6, dist. 1.16" according to the Leyton observations, was clearly separated on the mornings of the 25th, 27th, and 28th.
42 Orionis 5'11), dist. 2.95" (Knott). On the evening of the 26th the small star was occasionally quite free from the rings of light round the large star, a minute but bright point.
Trapezium. The fifth star has been generally very distinctly visible, and on several nights the sixth, though it has appeared to me minute in comparison with the fifth.
n Orionis. My object-glass fails to separate this close double star. The fine state of the atmosphere has enabled me to use very high powers, which show the two stars wedged together or overlapping, but without dividing them.
12 Lyncis and 36 Andromeda. My instrument separates and shows perfectly. I have also seen distinctly the components of 15 Pleiadum, 8, 14, dist. 5", and the 15mag. companion to B Aquarii.
Sun Spots. On the 30th ult. a feature was visible which I have never before seen so distinctly. A brilliantly luminous band bordered the f. side of a small spot which followed the large spot. Another spot of some size followed the small spot at a little distance. I observed this with one of Dawes's solar eye-pieces and powers from 90 to 300, with glasses of different tints. Other details in the spots were at times beautifully defined. It is probable that this spot and luminous band were observed in England with instruments of larger aperture.
I am, Sir, your obedient servant, Mentone, Alpes Maritimes :
D. A. FREEMAN. Feb. 7, 1866.
TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-As Mr. Huggins has been so kind as to favour us with the results of his experiments in binocular vision, I hope the matter may not be allowed to drop, but that others of your readers who have the means of doing so will make experiments in the same direction, as I feel assured that, as the binocular microscope brings out with singular beauty and solidity the structure of diatoms, and other objects examined by it, so the binocular eye-piece, adapted to the telescope, will give the same beautiful appearance to the nebula and lunar and planetary surfaces, bringing out the details with stereoscopic effect.
I think that the use of a binocular microscope as an eye-piece can hardly be expected to give a fair idea of what a binocular eye-piece would do, as the amount of light lost would be so very large from the rays having to pass through two objectives and so small an aperture.
I am surprised that the eye-piece I alluded to in my last letter is not known in England. It was invented by Mr. Tolles, an American maker; an account of it is given in Silliman's American Journal of Science for July 1864. It is also noticed in the Popular Science Review for October 1864, p. 127, to which I beg to refer your readers.
This eye-piece, I believe, was intended only to be used with the microscope, but as the whole effect of binocular vision is produced in the eye-piece, it appears admirably adapted for the telescope, or might be so fitted as to be used with either.
If Mr. Tolles's eye-piece is to be got in England, I should be glad to receive information on the subject.
I am, Sir, yours faithfully, Feb. 12, 1866.
C. S. HARRIS. P.S.-Perhaps Mr. Huggins would again direct Mr. Wenham's attention to this subject.
HIND'S CRIMSON STAR IN LEPUS.
TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-An inquiry was lately made by one of your correspondents in the Register for the place of this celebrated crimson star.
Mr. Hind, quoted by the late Admiral Smyth in Siderial Chromatics, p. 20, gives it Ř.A. (1850) 4h. 52m. 45s., and N.P.D. 105° 2'. Those who may not be furnished with equatorial or other arrangements for directing their at once on the ar, may readily find it
the S.D.U.K. star-maps, large or small.