Abbildungen der Seite

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, bat 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 cloud, 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 above 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 on a surface of snow, covered with a hoar.frost, exbibits, as I have latels 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.




pbænomena 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 retlection 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.



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 fartber from

it, &c.

A great many long and laborious calculations have been made by divers able philosophers and mathematicians, for the purpose of de. ducing 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 sheus either that the cal. culations have been established upon defective principles, or that not all the copcurring circumstances have bt en 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 travels so fast through the distance of 1000 or 2000 miles, that we cannot possibly perceive the time; therefore we may conclude that the er. 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.


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


[ocr errors]

* Principia. B. II. Prop. 50.
# Essay on Motion,

+ Phil. Trans. No 209.

Phil. Trans. No. 247.

[ocr errors]


* 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 dithculty of measuring time to a fraction of a second, that no very great degree of accuracy can be expected in measurements of this sort.

Derham observed, that the report of a cannon fired at the distance of thirteen miles froin him, did not strike his ears with a single sound, but that it was repeated five or six times close to each other. 66 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. I 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.

instantaneous, of which we cannot appreciate the height. Whereas in the former case, viz. at a distance, it is a grave sound, which may be compared to a determinate musical sound; and instead of being instantaneous, it begins softly, swells to its greatest loudness, and then dies away growling. Nearly the same thing may be observed with respect to a clap of thunder. Other sounds are likewise altered in quality by the distance.

Upon the whole, it appears that the velocity of sound is exactly the same, whether the sound be high or low, strong or feeble, whether it be the sound of a human voice, or the report of a cannon. But its velocity is sensibly altered by winds. If the wind conspires with the sound, viz. if it blows in the direction from the sounding body to the hearer, the sound will be heard sooner; and if the wind blows the contrary way, the sound will be heard later, than according to the rate of 1142 feet per second. In short, the velocity of the wind, in the former case, must be added to, and in the latter it must be subtracted from, that of the sound.. But the velocity of the air in the strongest wind is, per. haps, not equal to the twentieth part of the velocity of sound.

Heat and cold seem to make a very small alteration in the velo. city of sound; for sound appears to travel a little faster in summer than in winter.

Different altitudes of the barometer, as also different quantities of moisture in the air, seem to occasion a small alteration in the velocity of sound. But it is not in our power to determine what share of the effect is due to each of those causes.

Upon the whole it appears, that whatever increases the elasticity of the air, accelerates the motion, as also the intensity of sound through it, and vice versa. Or, in fluids of a determinate elas. ticity, whatever increases the density, diminishes the velocity of sound through them. Probably the velocities of sound through

• The knowledge of this fact will enable us to measure, pretty nearly, the velocity of the wind in certain cases ; for if a cannon be fired at a knowo distance from us, the report must reach us sooner when the wind blows from that place to us, and later when it blows the contrary way, than it will in calm weather; therefore, knowing what time it ought to reach us in calm weather, the difference between that time and the time observed in the above-mentioned cases of windy weather, is the time which the wind eroploys in passing through that distance.

« ZurückWeiter »