Abbildungen der Seite
[ocr errors]

in one second, or seven syllables in two seconds *. Therefore, when an echo repeats seven syllables, the reflecting object is 1142 feet distant; for sound travels at the rate of 1142 feet per second, and the distance from the speaker to the reflecting object, and again from the latter to the former, is twice 1142 feet. When the echo returns fourteen syllables, the reflecting object must be 2282 feet distant, and so on. A famous echo is said to be in Woodstock Park, near Oxford. It repeats seventeen syllables in the day, and twenty at night t. Another remarkable echo is said to be on the north side of Shipley church, in Sussex. It repeats distinctly, in favourable circumstances, twenty-one syllables f.

Therefore the farther the reflecting surface is, the greater num, ber of syllables the echo will repeat; but the sound will be en. feebled nearly in the same proportion, and at last the syllables cannot be heard distinctly.

When the reflecting object is too near, the repetition of the sound arrives at the ear, whilst the perception of the original sound still continues, in which case an indistinct resounding is heard, This effect may be frequently observed in empty rooms, passages, &c. especially because in such places several reflections from the walls to the hearer, as also from one wall to the other, and then to the hearer, clash with each other, and increase the indistinction.

If each of the vibrations of the air, which are occasioned by a certain sound, be performed in the same time that sound employs in going from the sounding body to the walls of a room, and thence to the hearer, then the sound will be heard with greater force. In short, by altering our situation in a room, and expressing a sound, or hearing the sound of another person, in different situations, or when different objects are alternately placed in the room, that sound may be heard louder or weaker, and more or less distinct. Hence it is, that blind persons, who are under the necessity of paying great attention to the perceptions of their sense of hearing, acquire the habit of distinguishing, from the sound even of their

* From the computation of short-hand writers it appears that a ready and rapid orator in the English language, probouoces from 7000 to 7500 words in an hour, viz. about 120 words in a minute, or two words in each second... Memoirs of Gibbon's Life.

+ Dr. Plot's Nat. Hist. of Oxfordshire, I Harris's Lex. Tech. Article Echo. VOL. IY.

2 N

own voices, whether a room is empty or furnished, whether the windows are open or shut; and sometimes they can even distinguish whether any person be in the room or not*.

[Cavallo's Philos.


Extraordinary Whispering-places, and Echoes. In a Letter from Robert Southwell, Esq. dated September 19, 1667. The best whispering place I ever saw was that at Gloucester: but in Italy, in the way to Naples, two days from Rome, I saw, in an ion, a room with a square vault, where a whisper could be easily heard at the opposite corner, but not at all in the side corner that was near to you.

I saw another, in the way from Paris to Lyons, in the porch of a common inn, which had a round vault; but neither of these were comparable to that of Gloucester; only the difference be. tween these last two was, that to this, holding your mouth to the side of the wall, several could hear you on the other side; the voice being more diffused. But to the former, it being a square room, and you whispering in the corner, it was only audible in the opposite corner; and not to any distance from thence as to distinction of words. And this property was common to each corner of the room, and not confined to one.

As to echoes, there is one at Bruxelles that answers fifteen times: but when at Milan, I went two miles from thence to a nobleman's palace. The building is of some length in the front, and has two wings jetting forward; so that it wants only one side of an oblong figure. About 100 paces before the house, there runs a small brook, and that very slowly; over which you pass from the house into the garden. We carried some pistols with us, and, firing one

• The famous Dr. N. Saunderson, professor of mathematics in the university of Cambridge, who bad been blind since he was ove year old, possessed such acuteness of hearing, that, as is related in the account of his life, “By his quickness in this sense, he not only distinguished persons, with whom be had ever once conversed, so long as to fix in his memory the sound of their yoice, but in some measure places also. He could judge of the size of a room into which he was introduced, of the distance he was from the wall: and if ever he had walked over a pavement in courts, piazzas, &c. which reflected a sound, and was afterwards conducted thither again, he could exactly tell whereabouts in the walk be was placed, merely by the note it sounded."

of them, I heard fifty-six reiterations of the noise. The first twenty were with some distinction ; but then, as the noise seemed to fly away, and answer at a greater distance, the repetition was so doubled, that you could hardly count them all, seeming as if the principal sound was saluted in its passage by reports on this and that side at the same time. Some of our company reckoned above sixty reiterations when a louder pistol was discharged.

[Phil. Trans. 1746.


Singular sympathetic action of two Pendulum Clocks on each


By Mr. John Elliscott, F.R.S.

The two clocks, on which the following observations were made, being designed for regulators, particular care was taken to have every part made with all possible exactness: the two pendulums were hung in a manner different from what is usual; and so disposed, that the wheels might act on them with more advantage. Upon trial, they were found not only to move with greater freedom than common, but a heavier pendulum was kept in motion by a smaller weight. They were in every respect made as near alike as possible. The ball of each of the pendulums weighed above 231b. ; and required to be inoved about 1° 5' from the perpendi. cular, before the teeth of the swing wheel would scape free of the pallets; that is, before the clocks would be set a-going. The weight to each was 31b. which would cause either of the pen. dulums in their vibrations to describe an arch of 3o. The two clocks were in cases, which shut very close, and placed sideways to each other, so near that when the peadulums were at rest, they were little more than about two feet asunder.

The odd phænomena observed in them were these : in less than two hours after they were set a-going, one of them, called No. 1, was found to stop; and when set a-going again, as it was several times, it would never continue going four hours together. As it had always kept going with great freedom, before the other clock, No. 2, was placed near it, this led Mr. E. to conceive its stopping must be owing to some influence the motion one of the pendulums had upon the other; and on watching them more, narrowly, the motion of No. 2 was found to increase as No. 1 diminished; and. at the time that No. 1 stopped, No. 2 described an arch of 5°; that is, two degrees more than it would have done, if the other had not bet n near it, and more than it moved in a short time after the other pendulum came to be at rest: this made Mr. E. imagine that they had a mutual influence on each other.

On this he stopped the pendulum of No. 2, leaving it quite at rest, and set No. 1 a-going, the pendulum describing as large an arch as the case would permit, viz. about 5°. In about twenty minutes after, he went to observe whether there was any motion commu. nicated to the pendulum No. 2; when, to his surprise, he found the clock going, and the pendulum to describe an arch of s', whereas at the same time No. 1 did not move 4o. In about half an hour after, No. 1 stopped, and the motion of No. 2 was in. creased to very near 5°. He then stopped No. 2 a second time, and set No. 1 a going, as before; and standing to observe them, be presently found the pendulum of No. 2 begin to move, and the motion to increase gradually, till in 17m. 40s. it described an arch of 2° 10', at which time the wheel discharging itself of the pallets, the clock went.

The arches of the vibrations continued to increase, till, as in the former experiment, the pendolum moved 5°; the motion of the pendulum No. 1, gradually decreasing all the while, as the other increased; and in three quarters of an hour after, it stopped,

He then left the pendulum of No. 1 at rest, and set No. 2 a going, making it describe an arch of 5°; it continued to vibrate less and less, till it described but about 3o; in which arch it con. tinued to move all the time he observed it, which was several hours. The pendulum of No. 1 seemed but little affected by the motion of No. 2.

Mr. E. tried these experiments several times over, without find. ing any remarkable difference. The freer the room was front any motion, as people's walking about in it, &c. he found the experiments to succeed the better; and once he found No. 2 set a-going in 16m. 20s. and No. 1 at that time stopped in 36m. 40s. Further observations and experiments concerning the two

Clocks abovementioned.

By the same. 'The seemingly different effects, which the two clocks had on each other, Mr. Ellicott accounts for as follows.

The manner in which the motion is communicated to the pen. dulum at rest, he conceives to be thus: as the pendulums are very heavy, when either of them is set a.going, it occasions by its vibraiions a very small motion, not only in the case the clock is fixed in, but, in a greater or lt»s degree, in every thing it touches; and this motion is communicated to the other clock, by means of the rail, against which both the cases bear. The motion thus communicated, which is too small to be discovered, but by means of some such like experiments as these, may be judged by many, insufficient to make so heavy a pendulum describe an arch of 2°, or large enough to set the work a-going; and indeed it would be $0, hui for the very great freedom with which the pendulum is made to move, arising from the manner in which it is hung. This appears from the very small weight required to keep it going, which, when the clock was first put together, was little more than ilb. And if the weight was taken off, and the pendulum made to swing 2°, it would make 1200 vibrations before it decreased half a degree, so that it would not lose the 3000th part of an inch in each vibration. Indeed if the weight was hung on, the friction would be increased, and the pendulum would not move quite so freely; but even in that case it was found to lose but litile more than the 2000th part of an inch, or about three seconds of a de. gree, in one vibration; and therefore if the motion cominunicated to it from the other, will make it describe an arch exceeding 3", the vibrations must continually increase till the work is set a going. And that the motion is communicated in the manner above supposed, is confirmed by the following experiments :

A prop was set agaiost the back of the case of No. 2, to prevent its bearing against the rail; and No. I was set a-going; then observing them for several hours, Mr. E. could not perceive the least motion communicated to No. 2. He then set both the clocks a.going, and they continued going several hours; but he could not fund they had any influence on each other. Instead of the prop against the back of the case, he put wedges under the bot, toms of both the cases, to prevent their bearing against the rail; and stock a piece of woont between them, just tight enough ta support its own weight. Then setting No. 1 a-going, the influence was so much increased, that No. 2 was set a-going in less than six minutes, and No. 1 stopped in about six minutes after. In order to try what difference would arise, is the clocks were fixed on a

« ZurückWeiter »