224 COMPENSATION PENDULUM. SECT. XXV. 



beautiful application of the laws of unequal expansion. 

 The oscillations of a pendulum are the same as if its 

 whole mass were united in one dense particle, in a cer- 

 tain point of its length, called the center of oscillation. 

 If the distance of this point from the point by which the 

 pendulum is suspended were invariable, the rate of the 

 clock would be invariable also. The difficulty is to neu- 

 tralize the effects of temperature, which is perpetually 

 increasing or diminishing its length. Among many con- 

 trivances, Graham's compensation pendulum is the most 

 simple. He employed a glass tube containing mercury. 

 When the tube expands from the effects of heat, the 

 mercury expands much more ; so that its surface rises 

 a little more than the end of the pendulum is depressed, 

 and the center of oscillation remains stationary. Har- 

 rison invented a pendulum which consists of seven bars 

 of steel and of brass, joined in the shape of a gridiron, 

 in such a manner that if by change of temperature the 

 bars of brass raise the weight at the end of the pendu- 

 lum, the bars of steel depress it as much. In general, 

 only five bars are used ; three being of steel and two a 

 mixture of silver and zinc. The effects of temperature 

 are neutralized in chronometers upon the same princi- 

 ple ; and to such perfection are they brought, that the 

 loss or gain of one second in twenty-four hours for two 

 days running would render one unfit for use. Accuracy 

 in surveying depends upon the compensation rods em- 

 ployed in measuring bases. Thus, the laws of the une- 

 qual expansion of matter judiciously applied have an 

 immediate influence upon our estimation of time : of 

 the motions of bodies in the heavens, and of their fall 

 upon the earth ; on our determination of the figure of 

 the globe, and on our system of weights and measures ; 

 on our commerce abroad, and the mensuration of our 

 lands at home. 



The expansion of the crystaline substances takes place 

 under very different circumstances from the dilatation 

 of such as are not crystalized. The latter become both 

 longer and thicker by an acession of heat, whereas M. 

 Mitscherlich has found that the former expand differ- 

 ently in different directions ; and in a particular instance, 

 extension in one direction is accompanied by contraction 



