308 CELESTIAL DYNAMICS. 



measurements, we are obliged to content ourselves with prob- 

 able estimates ; these estimates will carry the more weight 

 the less they are formed in favour of some preconceived opin- 

 ion. 



Considering what is known about the expansion and con- 

 traction of solids and liquids by heat and cold, we arrive at 

 the conclusion that for a diminution of 1 in temperature, 

 the linear contraction of the earth cannot well be less than 

 ioo^o6 tft part, a number which we all the more readily adopt 

 because it has been used by Laplace, Arago, and others. 



If we compare the capacity for heat of all solid and liquid 

 bodies which have been examined, we find that, both as re- 

 gards volume and weight, the capacity of water is the great- 

 est. Even the gases come under this rule ; hydrogen, how- 

 ever, forms an exception, it having the greatest capacity for 

 heat of all bodies when compared with an equal weight of 

 water. In order not to take the capacity for heat of the mass 

 of the earth too small, we shall consider it to be equal to that 

 of its volume of water, which, when calculated for equal 

 weights, amounts to 0*184.* 



If we accept Laplace's result, that the length of a day has 

 remained constant during the last 2500 years, and conclude 



* The capacity for heat, as well as the coefficient of expansion of mat- 

 ter, as a rule, increases at higher temperatures. As, however, these two 

 quantities act in opposite ways in our calculations, we may be allowed to 

 dispense with the influence which the high temperature of the interior of 

 the earth must exercise on these numbers. Even if, in consequence of 

 the high temperature of the interior, the earth's mass could have a capa- 

 city two or three tunes as great as that which it has from to 100, it 

 is to be considered, on the other hand, that the coefficient of expansion, 

 -.^-i--^ only holds good for solids, and is even small for them, whilst in 

 the case of liquids we have to assume a much greater coefficient : for mer- 

 cury between and 100, it is about six times as great. Especially great 

 is the contraction and expansion of bodies when they change their state 

 of aggregation ; and this should be taken into account when considering 

 the formation of the earth's crust. 



