402 Rev. C. -S. Lyman on the Pendulum Expermient 



rents into which it is constantly liable to be thrown. Few apart- 

 ments are closed perfectly tight; or if there be no currents from 

 this source, the motion of the observer in the room, his respira- 

 tion or the heat of his body, will constantly tend to disturb the 

 equilibrium of the air; and how great an effect on the delicale 

 movements of the pendulum the slightest current will have, may 

 be learned by breathing ever so gently towards such a pendulum 

 when in motion. A single breath will throw it into an elliptic 

 path and sensibly change its plane of oscillation. 



One great source of the elliptic motion so constantly observed 

 m these experiments undoubtedly is the almost insensible lateral 

 oscillation which the pendulum is apt to have at the moment of 

 being let off, notwithstanding the utmost care that may be taken 

 to extinguish it. Perhaps as good a way as any to secure perfect 

 quiescence at this moment, is to bring some smooth surface, like 

 that of an ivory scale, into contact with the point of the pendulum 

 index, while the other end of the scale restitig as the short arm 

 of a lever, on some solid support for a fulcrum, is gently pressed 

 by the finger. In this way the contact can be delicately re- j 



newed and broken till the slight friction on the point of the pen- * 



dulnm gradually brings it to rest. 



The common method of starting the pendulum by suddenly 

 letting loose the single thread attached to the loop which retains 

 it, is perhaps the best that can be employed. 



5. Phenomena observed. — When a pendulum, freely suspended 

 as before described, is put in vibration, the plane of oscillation 

 is seen gradually to change its position in reference to the points 

 of the compass, or to have a horizontal movement of rotation 

 around the vertical from left to right, or with the motion of the 

 hands of a watch. 



The m/e of this angular motion has been the point to which 

 most persons who have repeated the experiment, have directed 

 their attention. It has been observed to be different in different 

 places, and to correspond approximately to the amount assigned 

 by mathematical investigation, namely, the angular motion of 

 the earth on its axis multiplied by the sine of the latitude. A 

 further examination of this expression for the rate of motion will 

 be given further on. 



The rate of motion per hour, however, as given by experiment, 

 is by no means a constant quantity, even at the same place and 

 with the same pendulum, varying often in different trials a de- 

 gree, and sometimes two or three degrees^ from the mean, or from 



the theoretical amount. 



Mr 



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