Abbildungen der Seite
PDF
EPUB

ground. The sun was nearly setting, but shone extremely bright; I walked up to the cloud, and my shadow was projected into it. The head of my shadow was surrounded, at some dis. tance, by a circle of various colours, whose centre appeared to be near the situation of the eye, and whose circumference extended to the shoulders. This circle was complete, except what the sha. dow of my body intercepted. It exhibited the most vivid colours, red being outermost: as far as can be recollected, all the colours appeared in the same order and proportion that the rainbow presents to our view. It resembled, very exactly, what in pictures is termed a glory, around the head of our Saviour, and of saints: not indeed that luminous radiance, which is painted close to the head, but an arch of concentric colours, which is placed separate and distinct from it. As I walked forward, this glory approached or retired, just as the inequality of the ground shortened or length. ened my shadow. The cloud being sometimes in a small valley below me, sometimes on the same level, or on higher ground, the variation of the shadow and glory became extremely striking and singular.

To add to the beauty of the scene, there appeared, at a considerable distance, to the right and left, the arches of a white shining bow. These arches were in the form of, and broader than a rain. bow; but were not completely joined into a semicircle above, on account of the shallowness of the cloud. When my chaise came up, I could observe no peculiar appearance round the shadows of the postillion, horses, or chaise. But the postillion was alarmed, to an uncommon degree, by this very singular apparition : which, indeed, might excite terror, or delight, in the beholder, according to the disposition of mind with which it was viewed.

Several appearances have been described by philosophers, in some respects resembling what I saw, but not exactly the same. The arch in size, situation, and colour, was most exactly the glory represented in some pictures, and is manifestly the archetype whence it had been copied by a painter. Indeed such a phenome. non is well adapted to excite religious awe and reverence.

When I returned into the chaise, a bright radiance appeared close to its shadow, but no separate coloured circle was formed.

In order to investigate the cause of these curious appearances, on optical principles, it may be useful to note some peculiar cir. cumstances. The cloud was specifically heavier than the air of that region where it was placed : for, it descended, with considerable velocity, down the side of the mountain. It was very close and shallow, being, in part, compressed by its own weight; the air at that altitude being too rare to suspend it.

I have seen, at other times, but not frequently, clouds of the same appearance. On the 28th of November, 1780, I saw some clouds which exactly resembled those in which the glory was ob. served, but had not leisure to approach them. These were re. markably close, compact, and shallow on the mountain ; but, in the adjoining valley (of Mold), they were thin, rare, and deep. In the valley, the atmosphere and the clouds seemed to be of the same specific gravity. The cloud on the mountain had a shining brightness, where the sun shone upon it, but was extremely black where shaded by other clouds.

In some cases, the cloud in which a similar appearance has been observed, was thought to be composed of frozen particles. It probably was so in the present instance. For, some hours later, the same evening, being on horseback, and passing through a thick eloud, icicles had formed on my hair, which by the motion of riding produced a sound like the ringing of distant bells.

No coloured arch like a rainbow, I believe, has ever appeared in a hail or snow shower: the frozen drops are probably too opake, too distant, and too large, to exhibit such colours. But the proxi. mity and the minuteness of the frozen particles, in the cloud abore described, might probably allow the rays of light to be reflected, and refracted in a coloured circle. Experiments on thin frozen surfaces, in a prismatic form, or on small frozen particles of water, might successfully illustrate this curious subject. Glass incrusted with ice may afford some observations. And the sun shining op a surface of snow, covered with a hoar-frost, exhibits, as I have lately remarked, beautiful brilliant points of various colours, as red, green, blue, &c. reflected and refracted at different angles; which, by attentive observation, might perhaps explain the cause both of the glory, and of the bright arches above described.

Explanation of the Drawing.
CC. The white cloud.
AA. The shining arches.
SS. The shadow.
GG. The glory.

[Mem. of the Manchester Society.

CHAP. XLIX.

ON SOUNDS AND ECHOES.

Tue phænomena of sound have in many instances a strong re. semblance to those of light. Hook and Euler supposed both to consist in a mere vibration or impulse of ether, or of air. Newton contended for a peculiar luminous fluid, or matter of light, distinct from ether; and Epicurus for a peculiar sonorous fluid, or matter of sound, distinct from air; an opinion which Lamarck * appears to have revived in our own times. Light, as we have already seen, is subject to reflection and refraction, agreeably to definite laws; sound is subject to the same effects, and according to laws equally definite; and it is from this reflection and refraction that echoes or mock.voices, whispers, and confused murmurs, are produced, in the latter instances; as parhelia or mock-suns, rainbows, and halos in the former : the one is peculiarly affected by colours, on which the other altogether depends.

[EDITOR.

SECTION 1.

General Observations on the Nature of Sound, whispering

Domes, and Echoes.

Sound is propagated successively from the sounding body, to the places which are nearer to it, then to those that are farther from it, &c.

A great many long and laborious calculations have been made by divers able philosophers and mathematicians, for the purpose of de. duciag the velocity of sound through the air, from the known weight, elasticity, and other properties of air; but the results of such calculations differ considerably from each other; as also from the results of actual experiments ; which shew's either that the cal. culations have been established upon defective principles, or that not all the copcurring circumstances have been taken into the ac.

• Journal Physique, lxix. 397.

count. Therefore, without mentioning any thing farther with re. spect to those calculations, I shall immediately state the result of authentic and useful experiments,

Almost every body knows, that when a gun is fired at a consi. derable distance from him, he perceives the flash a certain time before be hears the report; and the same thing is true with respect to the stroke of a hammer, of an hatchet, with the fall of a stone, or, in short, with any visible action which produces a sound or sounds. This time which sound employs in its motion through the common air, has been measured by various ingenious persons. The principal and more general method has been, to measure (by means of a stop watch or a pendulum) the time which elapses between the appearance of the flash, and the hearing of the report of a gun fired at a certain measured distance from the observer; for light trarels so fast through the distance of 1000 or 2000 miles, that we cannot possibly perceive the time; therefore we may conclude that the ex. plosion of a gun takes place at the very same moment in which we perceive the flash.

In the first place it has been unanimously observed, that sound travels at an uniform rate ; viz. that it will go as far again in two seconds, as it will in one second ; that it will go three times as far in three seconds, or four times as far in four seconds, as it will in one, and so on. Therefore, in the above-mentioned manner of performing the experiment, if the distance, (in feet) between the cannon and the observer, be divided by the number of seconds elapsed between the perceptions of the flash and of the report, the quotient will shew the rate of travelling, or how many feet per second sound runs through.

This rate has been estimated differently by different persons, whose experiments have been performed at different times, in dif. ferent places, and with instruments more or less accurate, viz.

Feet

per Second, By Sir Isaac Newton, at the rate of.......... 968 + By the Hon. Mr. Roberts, at.... .. 1300 # By the Hon. Mr. Boyle, at...

1200
§ By Mr. Walker, at .....

,1338
* Principia. B. II. Prop. 50. of Phil. Trans. No 209.
Essay on Motion.

(Phil. Trans. No. 247.

* By Mersennus, at...

1474 + By the Florentine Academicians

1148 # By the French Academicians ...

1172 De Thury, Maraldi, and de la Caille

1107 || Flamstead, Halley, and Derham, at ........1142

Dr. Derham, as it appears by the account in the Philosophical Transactions, seems to have made the greatest number of accurate and more diversified experiments; therefore we may take his con. clusion, which coincides with those of Flamstead and Halley, as the nearest to the truth ; viz. that, in general, sound travels uni. formly through the atmospherical air at the rate of 1142 feet per second, or one mile in little less than five seconds; at least, this result cannot differ from the truth by more than fifteen or twenty feet. I But it will appear from the following paragraphs, and from the difticulty of measuring time to a fraction of a second, that no very great degree of accuracy can be expected in measure. ments of this sort.

Derham observed, that the report of a cannon fired at the distance of thirteen miles from him, did not strike his ears with a single sound, but that it was repeated five or six times close to each other. “ The two first cracks," he says, “ were louder than the third, but the last cracks were louder than any of the rest.-- And besides, in some of my stations, besides the multiplied sound, I plainly heard a faint echo, which was reflected by my church, and the houses adjacent."

This repetition of the sound probably originated from the reflec. tion of a single sound from hills, houses, or other objects, not much distant froin the cannon. But it appears from general ob. servation, and where no echo can be suspected, that the sound of a cannon, at the distance of ten or twenty miles, is different from the sound when near. In the latter case the crack is loud and

• Balistic. Prop. 39, + Exp. of the Acad. del Cimento. p. 141. # Du Hamel Hist. Acad. Reg.

They reckoned it equal to 173 toises, which are nearly = to 1107 feet English. See Mem. de l'Acad. for 1738, p. 128, &c.

# Phil. Trans. Jones's Abrid. vol. IV. p. 396.

1 According to Mr. Hale, the undulation of water is to the motion of sound, as one to 865.

« ZurückWeiter »