AIR AND LIFE. 197 



it forms a sort of protecting screen, that, by day, tempers the solar 

 heat by absorbing a portion of it and preventing it from scorching the 

 vegetation and the soil, and at night, conversely, prevents excessive 

 cooling of the earth's surface by radiation. It does not prevent the 

 passage of luminous calorific rays, but absorbs a large amount of the 

 dark thermic rays — whatever their source — and experiments by Tyn- 

 dall, and especially Pouillet, and others, have shown that the atmos- 

 phere, by reason of the vapor it contains, absorbs about a quarter of 

 the sun's heat, so that only three-quarters reach the earth proper. If 

 this screen did not exist, our summer days would be much hotter and 

 also much cooler. In the full glare of the sun the thermometer would 

 stand higher than it does, and in the shade the temperature would 

 be lower. We have an exact illustration of what would happen in 

 what occurs on high mountains, or in balloons at great height. The 

 higher we ascend the thinner becomes the layer of vapor interposed 

 between the sun and ourselves. Under such circumstances the sun is 

 scorching; its rays, nearly unopposed, exert a stronger influence upon 

 persons and things and heat them highly, while the surrounding air is 

 cold, as there is hardly any vapor to absorb solar heat. This fact has 

 been well observed by Professor Laugley during his ascent of Mount 

 Whitney, and all alpinists have had experiences more or less similar. 

 If, then, there were no vapor in the atmosphere, our summer days would, 

 as is the case in high altitudes, be torrid and frigid at the same time — 

 torrid in the sun, frigid in the Shade, where the thermometer would 

 certainly fall very low. 



At night atmospheric vapor moderates radiation. During the night 

 the earth gives off part of the heat it has received during the day, 

 and this heat radiates into interplanetary space. When the sky is 

 very clear and dry, radiation is considerable, and at all seasons a clear 

 night is colder than a cloudy one, and night is colder in high altitudes 

 where the overlying sheet of air and vapor is thin and rare, than in the 

 lowlands, with a thicker atmospheric layer overhead. Eadiation is 

 unavoidable, because the temperature of celestial space is exceedingly 

 low, j)robably inferior to 100° below zero (centigrade); but it is more 

 rapid, and offers greater intensity when the air is dry and contains but 

 a small quantity of vapor, because then the absorption by the atmos- 

 phere (by vapor, to be precise) of dark calorific rays radiated from the 

 earth's surface is very slight. If there were no aqueous vapor in the air, 

 a considerable cooling would begin as soon as the sun set, and such 

 cooling does occur on high mountains and at high levels — in Thibet, 

 for instance, which is both high and dry, and also in deserts, where the 

 atmosphere is generally dry. In Sahara, after the hottest days under 

 a scorching sun, the nights are generally very cool, and the thermom- 

 eter runs down some 30° or 40° in a few hours. Such radiation and 

 cooling must be very harmful, and most animals and plants could 

 certainly not endure it. Vapor thus exerts a most beneficial influence, 



