10 PLEASANT WA YS IN SCIENCE, 



element which behaved in this way would be weakened, if 

 we consider what would happen in the case of our own 

 earth, according as the air were simply moist but without 

 clouds, or loaded with cumulus masses but without cirrus 

 clouds, or loaded with cirrus clouds. For although there is 

 not in the case of the earth a central glowing mass like the 

 sun's, on whose rainbow-tinted spectrum the dark lines 

 caused by the absorptive action of our atmosphere could 

 be seen by the inhabitant of some distant planet studying 

 the earth from without, yet the sun's light reflected from 

 the surface of the earth plays in reality a similar part 

 It does not give a simple rainbow-tinted spectrum ; for, being 

 sunlight, it shows all the dark lines of the solar spectrum : 

 but the addition of new dark lines to these, in consequence 

 of the absorptive action of the earth's atmosphere, could 

 very readily be determined. In fact, we do thus recognize 

 in the spectra of Mars, Venus, and other planets, the 

 presence of aqueous vapour in their atmosphere, despite 

 the fact that our own air, containing also aqueous vapour, 

 naturally renders so much the more difficult the detection 

 of that vapour in the atmosphere of remote planets neces- 

 sarily seen through our own air. Now, a distant observer 

 examining the light of our own earth on a day when, though 

 the air was moist, there were no clouds, would have ample 

 evidence of the presence of the vapour of water; for the 

 light which he examined would have gone twice through our 

 earth's atmosphere, from its outermost thinnest parts to the 

 densest layers close to the surface, then back again through 

 the entire thickness of the air. But if the air were heavily 

 laden with cumulus clouds (without any cirrus clouds at 

 a higher layer), although we should know that there was 

 abundant moisture in the air, and indeed much more moisture 

 then there had been when there had been no clouds, our 

 imagined observer would either perceive no traces at all of 

 this moisture, or he would perceive traces so much fainter 

 than when the air was clear that he would be apt to infer that 

 the air was either quite dry, or at least very much drier than 



