85 



would have a tendency to cause the axis of the earth to describe 

 two small circles in the year amongst the stars. 



Now as the earth is an oblate spheroid, having its polar axis 

 26 miles shorter than its equatorial axis, or as it were a sphere of 

 8,000 miles diameter, with a belt of 1 3 miles thick at the equator, 

 sloping off to nothing North and South, the amount of such 

 motion caused by the impulse of a ring of 13 miles thick upon 

 the earth can be accurately calculated, it being proportional to 

 the relative weight of the belt and of the globe. The motion 

 calculated from the data of the average weight of the earth, and 

 its rigidity agrees with the observed amount of this motion. 



But if the earth were a liquid mass with only a solid shell, the 

 effect would be that the shell would, as it were, slide over the 

 liquid interior, and would not retard the oscillation caused by the 

 band to the same extent ; in other words, if the interior of the 

 earth were liquid, its precession would no longer be the same 

 that it is observed to be. But it may be said that under the 

 great pressure of the superincumbent rock, the interior might be 

 heated to any extent without becoming fluid. Experiments have 

 been made to test this (by, I think, Fairbairn, the Engineer) ; but 

 so far from confirming this idea, he found that granite, slate, and 

 other rocks which can be only melted at an intense white heat, 

 without pressure ; became fluid at a red heat when inclosed in 

 iron tubes and subjected to as much pressure as he could obtain 

 with a screAv. Therefore I conclude the earth is not liquid in its 

 interior, with a solid shell, and therefore the heat does not regu- 

 larly increase as you descend into it. 



If we observe the relations in which the thermal springs occiu', 

 we shall see at once that they seem to be divided into three 

 classes or varieties. 



We find springs that are evidently connected with active 

 existing volcanoes, and these are generally sulphurous, very hot, 

 and contain plenty of silica. 



We then have those that are closely connected with the basic 



