258 COMPENSATION PENDULUM. [SECT. xxv. 



ture the bars of brass raise the weight at the end of the pen- 

 dulum, 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 per- 

 fection 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 com- 

 pensation rods employed in measuring bases. Thus, the 

 laws of the unequal 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 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 direc- 

 tions ; and, in a particular instance, extension in one direction 

 is accompanied by contraction in another. The internal 

 structure of crystallized matter must be very peculiar, thus 

 to modify the expansive power of heat, and so materially to 

 influence the transmission of caloric and the visible rays of 

 the spectrum. 



Heat is propagated with more or less rapidity through all 

 bodies ; air is the worst conductor, and consequently mitigates 

 the severity of cold climates by preserving the heat imparted 

 to the earth by the sun. On the contrary, dense bodies, 

 especially metals, possess the power of conduction in the 

 greatest degree, but the transmission requires time. If a bar 

 of iron twenty inches long be heated at one extremity, the 

 caloric takes four minutes in passing to the other. The 

 particle of the metal that is first heated communicates its 

 caloric to the second, and the second to the third : so that the 



