GO 



"WELLS'S NATURAL rniLOSOPHT. 



Why do clocks 

 go faster in 

 winter tiian in 

 Eumnier ? 



How fire the 

 changes in the 

 length of pen- 

 dulums coun- 

 teracted ? 



Fig. 33. 



.\ 



G 



119. As heat expands, and cold contracts 

 all metals, a pendulum rod is longer in -warm 

 than in cold "sveathcr ; hence, clocks gain time 



in winter, and lose in the summer. 



As the smallest change in the length of a 

 pendulum alters the rate of a clock, it is highly 

 important, for the maintaining of uniform time, 

 that the expansion and contraction of pendu- 

 lums, caused bj changes in temperature, 

 should bo counteracted. For this purpose various contriv- 

 ances have been employed. The one most commonly em- 

 ployed at the present time is the mercurial pendulum, -which 

 is constructed 03 foUo\^-s : Tlio pendulima rod, A B, Fig. 33, 

 supports a glass jar, G H, containing mercury, inclosed in a 

 eteel frame-work, F C D E. "U'hen the vreather is warm, the 

 Fig 34. steel rod and frame-work expand, and thus in- 

 crease the length of the pendulum, and de- 

 press the center of oscillation. But, at the 

 same time, the mercury contained in the jar also 

 expands, and rises upwaid; and thus, by a 

 proper adjustment, the center of oscillation is 

 carried as far upward in one direction, as do-mi- 

 ward in the opposite direction, or the expansion 

 in both directions is equal, and the vftrations 

 of the pendulum remain uualterLd. Another form of pendu« 

 lum, called the "gridiron pendulum," Fig. 34, is composed of 

 rods of different metals, which expand unequally under the same 

 changes of temperature, and, by counteraction, keep the length 

 of the pendultmi constant. 



120. As the force of gravity determines how 

 long the pendulum shall he in falling down its 

 arc, and the time also of its rising in the op- 

 posite direction (since the ball of the pendu- 



Vim, as already stated, may be considered as a body de- 

 scending by its weight on a slope), it follows, that the time 

 of vibration of a pendulum will var}' as the attraction of 

 gravity varies. 



The same pendulum will vibrate more slowly at the equa- 

 tor than at the poles, because the attraction of gravitation is 

 less powerful at the equator. Tlierefore a pendulum to vi- 

 brate once in a second, must be shorter at the equator than 

 at the poles. Corresponding results take place wlien a pen- 

 dulum is carried to a moimtain-top, away from the center of the earth, wMcIi 



How do the 

 variations in 

 the force of 

 gravity aff>ict 

 the vibrntions 

 of a pendulum? 



Where will a 

 pendulum of a 

 given lencth 

 vibrate slow- 

 est, and where 

 the fastest ? 



