134 THE BORDERLAND OF SCIENCE. 



therefore I give results without entering into the 

 details of the processes by which they have been ob- 

 tained. It is the case, then, that the average daily 

 supply of light and heat on Mars (square mile for 

 square mile of his surface) is less than the supply on 

 the earth in the proportion of two to five. When he 

 is at his nearest to the sun, the daily supply amounts 

 to rather more than a half that received by the earth ; 

 but when he is at his farthest, the daily supply falls to 

 little more than one-third of the earth's. 



This is a very serious deficiency when rightly under- 

 stood. We must not content ourselves by comparing 

 it to the difference between the heat of a winter day 

 and a summer day. We often have to endure for 

 several days in succession a much greater degree of 

 cold than would follow from the mere reduction of the 

 sun's ordinary heat to one-third its present value, and 

 the deficiency is not destructive to life. But it would 

 be quite another matter if the whole supply of light 

 and heat to the earth were reduced in this proportion. 

 It must be remembered that to that supply we owe 

 the continuance of all the forms of force, including 

 vitality, on the whole earth. ' The sun's rays,' said 

 John Herschel, in 1833,* 'are the ultimate source of 

 almost every motion which takes place on the surface 

 of the earth. By its heat are produced all winds and 

 those disturbances in the electric equilibrium of the 

 atmosphere which give rise to the phenomena of 



* Before the notion had suggested itself to Stephenson, to whom it 

 is commonly referred. 



