148 DISCOURSE ON THE STUDY 



total annual amount of solar radiation will deter- 

 mine, cceteris paribus, the general climate of the 

 earth. Now, it is not difficult to show that this 

 amount is inversely proportional to the minor axis 

 of the ellipse described by the earth about the 

 sun, regarded as slowly variable ; and that, there- 

 fore, the major axis remaining, as we know it to be, 

 constant, and the orbit being actually in a state of 

 approach to a circle, and, consequently, the minor 

 axis being on the increase, the mean annual amount 

 of solar radiation received by the whole earth must 

 be actually on the decrease. We have here, there- 

 fore, an evident real cause, of sufficient univer 

 sality, and acting in tJie right direction, to account 

 for the phenomenon. Its adequacy is another 

 consideration. * 



(141.) Whenever, therefore, any phenomenon pre- 

 sents itself for explanation, we naturally seek, in 

 the first instance, to refer it to some one or other 

 of those real causes which experience has shown to 

 exist, and to be efficacious in producing similar 

 phenomena. In this attempt our probability of 

 success will, of course, mainly depend, 1st, On the 

 number and variety of causes experience has placed 

 at our disposal ; 2dly, On- our habit of applying them 

 to the explanation of natural phenomena ; and, Sdly, 

 On the number of analogous phenomena we can . 

 collect, which have either been explained, or which 

 admit of explanation by some one or other of those 

 causes, and the closeness of their analogy with that 

 in question. 



* The reader will find this subject further developed in a 

 paper lately communicated to the Geological Society. 



