112 THE PKOGRESS OF SCIENCE n 



deduction from the facts of perturbation ; and the 

 immediate confirmation of that determination, in 

 the year 1846, by observers who turned their 

 telescopes into the part of the heavens indicated 

 as its place, constitute a remarkable testimony of 

 nature to the validity of the principles of the 

 astronomy of our time. In addition, so many new 

 asteroids have been added to those which were 

 already known to circulate in the place which 

 theoretically should be occupied by a planet, 

 between Mars and Jupiter, that their number now 

 amounts to between two and three hundred. I 

 have already alluded to the extension of our 

 knowledge of the nature of the heavenly bodies by 

 the employment of spectroscopy. It has not only 

 thrown wonderful 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 elementary bodies. 



The application of the generalisations of 

 thermotics to the problem of the duration 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 CJ>IK lis 

 of the history of the universe ; and the demon st ra- 

 tion of the competency of the great seeular 

 changes, known under the general name of the 

 precession of the equinoxes, to cause corresponding 

 modifications in the climate of the two heiui- 



