PoLYTECHmo Association Progeebinos. 809 



been acoustomed to putting in our pendulum springs too long. 

 You' will see at Jones', in Courtlandt street, quite a good sized 

 tower clock of German make, with a pendulum spring not to exceed 

 a quarter of an inch long. But that is an opposite extreme. 



A long, heavy pendulum seems indispensable for time-keeping. 

 It should be as long as convenient, and as heavy as the clock is 

 able to keep in motion without requiring power out of proportion 

 to its size. It must have a controlling influence, a power in itself. 

 It must rule, and not be ruled. 



The nigher the center of gavitation is to the end, the more effective. 

 Therefore, I have a great dislike to those massive grid-iron arrange- 

 ments, which bring the length of seconds' pendulum to forty-seven 

 or eight inches, with a ten-pound ball. A mercurial pendulum that 

 beats seconds, will measure about forty-fom* inches. If the rod is 

 heavy, they will measure more; if light, a little less. Some pen- 

 dulums have been made to vibrate in four seconds. These would 

 be fifty-two feet three inches from the point of suspension to center 

 of oscillations, or about fifty-five feet whole length. Here another 

 objection arises, for unless the rod is very large, the impulse causes 

 a tremor, and consequent loss of power and motion. When St. 

 George's clock was first set running, the rod had a decided tremor, 

 but this was entirely obviated by a piece of wire ten feet long, put 

 on each side, securely fastened at each end, and braced out in the 

 center to six inches. This increased the motion and the speed ten 

 seconds a day. 



The point or center of oscillation is considered as fixed and immu- 

 table, and is the standard from which all calculations on pendulums 

 proceed. This point is estimated at about 39,2 for this latitude for 

 a seconds' pendulum. Now here comes a point that puzzles me. 

 The computations to get at this center of oscillation, is based on 

 laws of gravity. Now I cannot see why the two do not agree. 

 Reid tells us that the center of gravity must be above the center 

 of oscillation, the latter being a fixed point, the other not. I think 

 it is conceded that if a ball were suspended by an imaginary line, 

 the two points would agree, but as soon as we add a rod of any 

 description, the gravity center raises in certain proportions. In a 

 wooden rod pendulum, weighing ten pounds ten and a half ounces, 

 of which the ball weighed ten pounds, Reid found the center of 

 gravity was only one-tenth above the center of oscillation, while 

 a gridiron pendulum, the ball of which weighed sixteen pounds 

 three and a half ounces, and in all, with the rods, nineteen pounds 



