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400 Rev. C. S, Lyman on the Pendulum Experiment 



is to take place in a horizontal position, and after attaching a 



small weight to the extremity of it, observing whether the weight 

 bends it down always to the same extent while the wire is tnrned 

 around on its axis. If any error in the movement of the plane 

 of vibration of the penduhim is due to a different flexibility of 

 the wire in different directions, the effects of it may be elimina- 

 ted by so constructing the support to which the wire is attached 

 that it can be shifted in azimuth during the experiments.* 



2. Length of Pendulum wire and iveight of ball— The 

 greatest length hitherto employed was that of 220 feet, at the 

 Pantheon at Paris. The pendulum at Bunker Hill Monument 

 was 210 feet long. That used at Providence was 97 feet, that at 

 New Haven 71 feet, while in Great Britain and on the continent, 

 few of the pendulums used were over 50 or 60 feet in length. In 

 many cases they have been less than ten feet; and even with 

 these the change in the direction of the plane of oscillation is 

 clearly shown. The weight of the ball employed has been very 

 various, ranging from two to eighty or ninety pounds; and it 

 has usually been made of lead, which, on account of its great 

 specific gravity, is better adapted to this purpose than any other 

 of the ordinary metals. 



Iron has been suspected by some to be an unsuitable material 

 on account of its magnetic qualities. Mr. Bunt at Bristol, from 

 some experiments with an iron weight, in which he was troubled 

 with very anomalous results, inferred that it was owing to the 

 influence of magnetic currents. But it is difficult to conceive 

 how the fluctuating irregularities which he observed, could be 

 produced by such a cause ; and, besides, M. Bravais, in his exper- 

 iments on the revolutions of a conical pendulum, employing a 

 weight composed of mercury enclosed in some experiments in a 

 copper shell and in others in one of iron, could detect no difier- 

 ence in the results, and concluded from his very delicate experi- 

 ments, that the iron exerted no sensible influence whatever. 



It is obvious that the longer the wire and the heavier the ball 

 of a pendulum, the greater will be the probability of accurate re- 

 sults. For the resistance of the air being proportional to the 

 square of the velocity of a body moving in it, this resistance when 

 the mass of the body is great and its motion slow, will have 

 comparatively but little effect on the direction of the vibrations. 

 This is the main reason why short pendulums do not succeed as 

 well as long ones. 



3. Length of the arc of vibration, — It seems to have been an 

 object with most of the European experimenters to give the 



* Some h^ completed the suspension at top by means of softened catgut or 

 eilk cord, as at York, England, by Mr. Pbillipg. But the great eUipticity of the patii 



of hia nenJnhim and thw irrpornlnritipa I'n ih(^ mcmUa i^n nn+. stroni/lv rCCOjlUnen^ 



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