THE PROBLEMS OF ASTRONOMY. T9 



least involved Concrete Sciences. The astronomer 

 seeks to explain the Solar System. He does not stop 

 short after generalizing the laws of planetary move 

 ment, such as planetary movement would be did only 

 a single planet exist ; but he solves this abstract-con 

 crete problem, as a step to wards solving the concrete 

 problem of the planetary movements as affecting one 

 another. In astronomical language, &quot;the theory of 

 the Moon 5 means an interpretation of the Moon s 

 motions, not as determined simply by centripetal and 

 centrifugal forces, but as perpetually modified by 

 gravitation towards the Earth s equatorial protuber 

 ance, towards the Sun, and even towards Yenus 

 forces daily varying in their amounts and combina 

 tions. Nor docs the astronomer leave off when he has 

 calculated what will be the position of a given body 

 at a given time, allowing for all perturbing influences ; 

 but he goes on to consider the effects produced by re 

 actions on the perturbing masses. And he further 

 goes on to consider how these mutual perturbations 

 of the planets cause, during a long period, increasing 

 deviations from a mean state; and then how compen 

 sating perturbations cause continuous decrease in the 

 deviations. That is, the goal towards which he ever 

 strives, is a complete explanation of these complex 

 planetary motions in their totality. Similarly with 

 the geologist. Ho docs not take for his problem only 

 those irregularities of the Earth s crust that are 

 worked by denudation ; or only those which igneous 



