76 



duced by solar radiation in connexion with the modifying effects 

 of the atmosphere : we have no reason to suppose that climate 

 has ever been governed by any other source of heat. In order 

 to understand the general nature of the present climates from 

 pole to pole, let a convex lens be exposed to the rays of the sun, 

 the rays will be concentrated into the focus, and cause intense 

 heat ; the degree of heat depends on the number of rays col- 

 lected and concentrated into the focus. If we expose a small 

 point to this focus it would be intensely heated ; but if, in- 

 stead of a point, we place a large body in the focus, the heat 

 would be diminished, and the nearer we approach the lens the 

 less the heat. 



Our globe occupies the focus of the atmospheric lens, illus- 

 trated in Plate XX., and thus prevents the concentration of the 

 sun's rays into one point. In the equatorial region, when the 

 sun is in the zenith, if we ascend to the height of 20,000 feet, 

 instead of finding it hotter as we rise, we get into colder regions, 

 i. e. perpetual snow. Hence it follows that one planet may be 

 near the sun, and another placed at the confines of the solar sy- 

 stem, and yet both possessing equal temperature on their sur- 

 faces, by proportioning the diameter of their atmospheric lenses 

 to that of their respective bodies. Therefore to say that Venus 

 must be intensely hot and that Saturn is in the region of eternal 

 snow, is a mere assumption, unsupported by analogy. Some 

 attempts have been made to show how the globe may have be- 

 come hot and cold by its probable exposure to the effects of 

 intense stellar radiation in passing through the milky way ; but 

 such speculations are flights of the fancy, and unworthy of atten- 

 tion. There has been too much dependence placed on celestial 

 observations already for the progress of geology. If we would 

 speculate to any purpose on a former state of our globe, and 

 on the succession of events which from time to time have 

 changed the condition and form of its surface, we must confine 

 our principal inquiries to the results or effects of terrestrial 

 physics, and not attempt to solve the problems by reference to 

 celestial objects ; for if we differ so materially on those points 

 which we can handle, it is not probable that we could decide 

 by referring to objects so much beyond our reach. 



We shall give a few examples of some points used by geolo- 

 gists for their guidance in theoretical researches, founded on 



