IN THE LAST HALF-CENTURY. 113 



copy. It has not only thrown wonder- 

 ful light upon the physical and chemical 

 constitution of the sun, fixed stars, and 

 nebulae, and comets, but it holds out a 

 prospect of obtaining definite evidence 

 as to the nature of our so-called element- 

 ary bodies. 



The application of the generalisations its rela- 

 of thermotics to the problem of the dura- geology. 

 tion of the earth, and of deductions from 

 tidal phenomena to the determination of 

 the length of the day and of the time of 

 revolution of the moon, in past epochs of 

 the history of the universe ; and the dem- 

 onstration of the competency of the great 

 secular changes, known under the general 

 name of the precession of the equinoxes, 

 to cause corresponding modifications in 

 the climate of the two hemispheres of our 

 globe, have brought astronomy into inti- 

 mate relation with geology. Geology, in 

 fact, proves that, in the course of the past 

 history of the earth, the climatic condi- 



