272 EXPANSION. SECT. XXVII. 



The dilatation of substances by heat, and their contraction 

 by cold, occasion such irregularities in the rate of clocks and 

 watches as would render them unfit for astronomical or nautical 

 purposes, were it not for a very 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 certain point of its length, called the centre 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 neutralize the 

 effects of temperature, which is perpetually increasing or dimi- 

 nishing its length. Among many contrivances, Graham's com- 

 pensation 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 centre of oscillation remains stationary. 

 Harrison 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 pendulum, 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 

 principle ; 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 employed in measuring 

 bases. Thus, the laws of the unequal expansion of matter judi- 

 ciously applied have an immediate influence upon our estima- 

 tion 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 crystalline substances takes place under 

 very different circumstances from the dilatation of such as are 

 not crystallized. The latter become both longer and thicker by 

 an accession of heat, whereas M. Mitscherlich has found that the 

 former expand differently in different directions ; and, in a par- 

 ticular instance, extension in one direction is accompanied by 

 contraction in another: for example, Iceland spar is dilated 



