more solid floor, he placed them, exactly in the same manner as in the last experiment, on the stone pavement under the piazzas of the Royal Exchange, and stuck the piece of wood between them, as before; and setting No. 1 a-going, the only difference was, that it was fifteen minutes before No. 2 was set a-going, and No. 1 continued going near half an hour before it stopped.

From these experiments Mr. E. thinks it plainly appears, that the pendulum which is put in motion, as it moves towards either side of the case, makes the pressure on the feet of the case to be unequal, and, by its weight, occasions a small bearing or motion in the case on that side towards which the pendulum is moving; and which, by the interposition of any solid body, will be com municated to the other clock, whose pendulum was left at rest. The only objection to this, he conceives, is the different effects which the two pendulums seemed to have on each other. But this he hopes to explain to satisfaction.

For, notwithstanding these different effects, he soon found, by several experiments, that the two clocks mutually affected each other, and in the same manner, though not with equal force; and that the varieties observed in their actions on each other, arose from the unequal lengths of their pendulums only.

For, on moving one of the clocks to another part of the room, and setting them both a going, he found that No. 2 gained of No. 1, about 1m. 30s, in twenty-four hours. Then fixing both against the rail, as at first, he set them a-going, and made the pendulums to vibrate about 4°; but he soon observed that of No. 1 to in. crease, and that of No. 2 to decrease; and in a short time it did not describe an arch large enough to keep the wheels in motion. In a little time after it began to increase again, and in a few mi. nutes it described an arch of 2o, and the clock went. Its vibra tions continued to increase for a considerable time, but it never vibrated 4°, as when first set a-going. While the vibrations of No. 2 increased, those of No. 1 decreased, till the clock stopped, and the pendulum did not describe an arch of more than 1° 30′. It then began to increase again, and No. 2 decreased, and stopped a second time, but was set a-going again, as before. After this No. 1 stopped a second time, and the vibrations continued to de. crease till the pendulum was almost at rest. It afterwards in. creased a smaller matter, but not sufficiently to set the work a. going. But No. 2 continued going, its pendulum describing an

arch of about 3°.

Finding them to act mutually and alternately on each other, Mr. E. set them both a-going a second time, and made the pendulums describe as large arches as the cases would permit. During this experiment, as in the former, he sometimes found the one, and at other times the contrary pendulum to make the largest vibration. But as they had so large a quantity of motion given them at first, neither of them lost so much during the period it was acted on by the other, as to have its work stopped, but both continued going for several days, without varying one second from each other; though when at a distance, as was before observed, they varied 1m. 35s. in twenty-hours. While they continued thus going together, he compared them with a third clock, and found that No. I went 1m. 17s. faster, and No. 2 went 19s. slower, than they did when placed at a distance, so as to have no influence on each other.

On altering the lengths of the pendulums, the period in which their motions increased and decreased, by their mutual action on each other, was changed; and would be prolonged as the pendulums came nearer to an equality, which, from the nature of the action, it was reasonable to expect it would. This discovers the reason why the pendulum of No. 2, when left at rest, would be set a-going by the motion of No. 1; whereas if No. 1 was left at rest, it would not be set a-going again by the motion of No. 2.

For he found, by several experiments, that the same pendulum, when kept in motion by a weight, would go faster, than when it only moved by its own gravity. On this principle, which may easily be accounted for, it follows, that during the time in which the shortest pendulum, No. 2, was only acted on by No. 1, it would move slower, and the times of its vibrations approach nearer to an equality with those of No. 1, than after it came to be kept in motion by the weight; and by this means the time which No. 1 would continue to act on it, would be prolonged, and be more than was required to make the pendulum describe an arch sufficient to set the work a-going. But, on the contrary, while the pendulum of No. 1, which was the longest, was only acted on by No. 2, as it would move slower, the difference of the times of the vibrations would be increased; and consequently the time which No. 2 would continue to act on it, would for this cause be shortened, so that before the pendulum of No. 1 would describe an arch sufficient to set the work a-going, the period of its being

acted on would be ended, and it would begin to act on No. 2; at which time its vibrations would immediately decrease, and con. tinue to do so till it came to be almost at rest. And thus it would continue, sometimes to move more, and at other times less, but never sufficiently to set the clock a-going.

[Phil. Trans. 1739.

In summer, the thermometer being at 20°, Bianconi observed, that seventy-six vibrations of the pendulum elapsed while a sound passed over thirteen miles; in winter seventy-nine seconds, the thermometer being at 1.2°. In a cloud or mist 155" elapsed while the sound passed and repassed. Hence the air should expand for 21.2°, or for 1' of the thermometer employed, probably Reaumur's, which is for 1° of Fahrenheit.

Chladni infers, from the longitudinal vibration of different sub. stances a velocity of 7800 feet in a second, in tin; 9300 in silver; 12500 in copper; 17500 in glass and iron; from 11000 to 18000 in wood; from 10000 to 12000 in tobacco pipes. His observa. tions are fully confirmed by calculations from different grounds. According to the elasticity of fir, as inferred from an experiment of Mr. Leslie, the velocity of an impulse should be 17800.

Most, perhaps all, gases, are mediums of sound, and by their change of temperature produce sound. This is particularly the case with hydrogen gas. Aqueous vapour, that of ether, and of phosphorous acid, produce a similar, but slighter, effect.

[Bianconi. Brugnatelli. Chladni. Delarive. Young.

THE END OF VOL. IV.

R. WILKS, Printer, Chancery-lane.