on a new Theory of Dew. 493 



interior of Persia is characterized by the absence of dew. 

 In this region there are no rivers of any magnitude, and no 

 rain falls from May to the end of November. But on 

 approaching the Persian Gulf nocturnal dews are heavy and 

 the coverings of beds on the house roofs become saturated 

 with wet. In the African desert of Sahara, the traveller 

 Denham suffered from the dryness of the air until within a 

 certain distance of Lake Tchad, where, though there was no 

 appearance of water at any part of the horizon, the dews were 

 so abundant as to wet the clothes of people outside the tent. 

 When Dr. J. D. Hooker was in East Nepal he noticed that 

 the sun in many places did not reach the bottom of the valleys 

 until 10 a.m., and was off again by 3 p.m., while the radiation 

 towards a clear sky was so powerful that dew frequently 

 formed in the shade throughout the day. Such, too, was the 

 clearness of the sky that at night our traveller found the 

 upper blanket of his bed coated with moisture from the rapid 

 abstraction of heat by the tarpaulin of his tent, which had 

 become frozen by its own radiation. 



In equatorial regions, where the nights are long, dews are 

 so abundant that Humboldt compares their effects to those of 

 rain, and they become more and more abundant in ap- 

 proaching the equator ; whereas in the great assemblage of 

 islands known as Polynesia, the dew is feeble or absent, in 

 consequence of the trade and other warm winds from the sea 

 preserving a nearly uniform temperature. 



There is such a vast consensus of scientific opinion in 

 favour of the received theory of dew, that any attempt to set 

 it aside in favour of another must be supported by the 

 strongest experimental evidence. And yet some of Mr.. 

 Aitken's proofs seem to bear testimony to the received theory 

 rather than to the one now advocated. For example, when 

 Mr. Aitken exposes a turf six inches square to the air in a 

 scale-pan and finds it to have lost weight, he does not touch 

 the question whether the vapour that forms dew is not already 

 in the air before it is condensed and deposited. He only 

 proves that a moist soil is constantly giving off vapour under 

 a clear sky. Snow and ice behave in like manner: 100 

 grains of light snow have been known to lose 60 grains in 

 weight during one night when the temperature was below 25° ; 

 and Patrick Wilson, exposing various objects on a balanced 

 board (named by him a " snow-scale "), noticed that as they 

 cooled down by radiation, they became covered with hoar- 

 frost and increased in weight, and the vapour which supplied 

 the rime was derived from the air, seeing that the scale-board 

 was elevated 24 feet above the ground. 



