SECT, xxv.] EXPANSION. 257 



of expansion of solids varies at their transition to liquidity, 

 and that of liquidity is no longer equable near their change 

 to an aeriform state. There are exceptions however to the 

 general laws of expansion ; some liquids have a maximum 

 density corresponding to a certain temperature, and dilate 

 whether that temperature be increased or diminished. For 

 example water expands whether it be heated above or cooled 

 below 40. The solidification of some liquids, and especially 

 their crystallization, is always accompanied by an increase 

 of bulk. Water dilates rapidly when converted into ice, 

 and with a force sufficient to split the hardest substances. 

 The formation of ice is therefore a powerful agent in the 

 disintegration and decomposition of rocks, operating as one 

 of the most efficient causes of local changes in the structure 

 of the crust of the earth ; of which we have experience in 

 the tremendous eboulemens of mountains in Switzerland. 



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 nau- 

 tical 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 diminishing its length. Among many contri- 

 vances, 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 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 tempera- 



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