THE SMITHSONIAN INSTITUTION. 195 



The amount of moisture in the atmosphere has a great influence in 

 modifying the heating power of the sun's rays, as well as in the 

 radiation of heat during the night into space. We often find two days 

 in summer equally free from clouds and equally calm, while the one 

 may be oppressively warm, and the other comparatively cool. This 

 does not arise from a difference in the power of the sun's rays, but from 

 the quantity of moisture in the atmosphere. 



The influence of moisture in tempering the sun's rays is a remark- 

 able fact and well worthy of father investigation. When tlie dew 

 point is high, or, in other words, when the air is filled with moisture, 

 the radiation from the earth is prevented, and the temperature of the 

 night remains almost as high as that of the day ; when the dew 

 point is low, the sun's rays pass, without absorption, to the earth, and 

 impart little of their heat directly to the air. The medium dew points 

 are, therefore, most favorable to extreme heat in the atmosphere, and 

 the greater heat beyond the tropics is probably owing to this cause. 



The fact that the amount of moisture in the air regulates the tempe- 

 rature of the nights has not received the attention which it deserves. 

 I shall hereafter show that the hygrometrical condition of the atmo- 

 phere throws a very considerable amount of light on the action of 

 Nortli American and European storms; and therefore I am more anxious 

 to draw your particular attention to the relation between the dew point 

 of the vapor of the air and the night temperatures, because this is the 

 only means which I have of indicating the hygrometrical conditions 

 connected with the storms of the United Stales. 



It seems to be a law which holds in general over the world that the 

 temperature of the air at sunrise during calm nights, at a certain dis- 

 tance from the ground, falls a fittle below the dew point of the air 

 during the preceding day. The mean temperature of the^air at sun- 

 rise, therefore, approximates very closely to the mean dew point. The 

 great amount of moisture in the air within the tropics is the cause of 

 the warm and brilliant nights. Radiation from the air and ground, 

 under these conditions, seems to lose its power. On the other hand, 

 travellers in all parts of the world inform us, incidentally, as to the 

 connection between dry air and cold nights. Mr. Inglis, in his travels 

 through Spain, relates that he was oppressed by the hot rays of the sun 

 in the valley of Granada while the hoar frost was lying white in the 

 shade. Eastern travellers in the deserts often complain of the broiling 

 heat of the air during the day, and of its chill temperature at night. 

 Beautiful allusions to the same law are also found in Scripture; many 

 of you will recollect that one of the greatest hardships which Jacob 

 experienced while he tended Laban's flocks was, that through the 

 "drought by day and the frosts by night sleep departed from his eyes." 

 On the other hand, the moisture from the Atlantic in summer allows 

 the air to retain its heat in the valley of Chamouni, in Switzerland, so 

 that grapes ripen in the immediate proximity of the glaciers which 

 descend from the Alps. At Bergen, in Norway, the same element 

 allows the cherry-tree to mature its fruit where you can pluck it and 

 throw the stones upon the broad mass of ice which slowly descends 

 from the mountains. 



Those days during summer in the United States on which the sky 



