[ 99 ] 



XI. On the Influence of the Earth's Internal Structure on the 

 Length of the Day. By Henry Hennessy, M.R.I.A.y Pro- 

 fessor of Natural Philosophy in the Catholic University of 

 Ireland^. 



THE period of a complete revolution of tlie earth around its 

 axis of rotation, depends not only on the dimensions of the 

 mass, but also on the distribution of the particles of which it is 

 composed. The variations which in the course of ages may pos- 

 sibly take place in the distribution of these particles will there- 

 fore tend to produce some change in the length of the day. The 

 question of the secular variation of the earth^s velocity of rota- 

 tion has been already, to a certain extent, examined by Laplace f; 

 but his investigation has regard solely to the contraction of the 

 dimensions of the globe considered as a slowly cooling solid. If 

 the interior of the earth is in a state of fluidity from heat, or in 

 other words, if it consists of a solid exterior shell filled with a 

 nucleus of matter in a state of fusion, from which state the por- 

 tion now solid had gradually passed to its present condition, the 

 inquiry assumes a different shape. Hitherto this, as well as 

 every other question connected with the general structure and 

 rotation of the earth, has been treated on the assumption that 

 the portions composing the fluid underwent no change in their 

 positions on entering into the solid state. This assumption 

 formed not only the basis of the inquiries of Laplace, but even 

 of more recent investigators, who professed to consider the purely 

 physical conditions of problems relating to the Structure of our 

 planet in a far more complete manner. The change of state of 

 the matter composing the interior of the earth in passing from 

 fluidity to solidity seems to have been thus entirely overlooked, 

 although a little reflection might have suggested that such a 

 change would possibly influence its structure to a very consider- 

 able extent. 



In the first part of my " Researches in Terrestrial Physics %" 

 the necessity of attending to this circumstance was distinctly 

 pointed out, and the superfluous nature of the contrary assump- 

 tion was formally declared. Experiments were quoted in the 

 second part confirmatory of my views on this point, which I 

 believe have never since been called in question. When we 

 reflect on the remarkable results of the researches of Professor 

 Bischof, M. Ch. Sainte-Claire Deville, and M. Delesse, upon the 

 contraction of the principal materials of the solidified crust of 

 the earth in passing successively from the fluid state through 



* Communicated by the Author. ^^^^ i^'^ ' :_^' -r |> 

 u {'■ vi- f Mecanique Celeste, livre xi. - <? iinfrr-ij^ris -rli^v^n g^jgd 

 t Philosophical Transactions, 1851, part 2, p. 495. 

 H2 



