30-i Analyses of Books. [Oct. 



from radiation; and as air is a veiy bad conductor, the heat thus 

 lust cannot be supplied by the ambient atmosphere. Therefore the 

 temperature of bodies at the surface of the earth will become and 

 continue colder than the air during the whole night. But this will 

 happen only when the sky is clear, and the atmosphere still. If the 

 sky be covered by clouds, these opaque bodies will radiate back as 

 much heat as they receive from the bodies on the surface of the 

 earth ; so that the temperature of these bodies will not sink. If the 

 night be windy, the agitation of the atmosphere will in some mea- 

 sure make up for its bad conducting power. New and warmer por- 

 tions of air coming continually in contact with the bodies on the 

 surface of the earth will be continually supplying them with heat, 

 and thus prevent their temperature from sinking from radiation. 



Thus we see a surticient reason why the temperature of bodies on 

 the surface of the earth in clear and calm nights becomes consider- 

 ably lower than that of the surrounding atmosphere. But what is 

 the reason that the temperature of some bodies becomes much lower 

 than that of others in their immediate vicinity ? The discoveries of 

 Mr. Leslie enable us to answer that question in a satisfactory 

 manner. It is because some bodies radiate much more heat than 

 others. Metals are the bodies which radiate heat worst, of course 

 they will be less cooled by radiation than other bodies, and less dew 

 will of course condense on them, as is the matter of fact. Gold, 

 silver, copper, and tin, are the metals that radiate heat worst, and 

 they are the metals upon which dew is least apt to form. Filament- 

 ous substances, paper, grass, radiate heat copiously : hence they 

 become speedily colder than the air, and dew forms upon them in 

 abundance. 



Such is the theory of the formation of dew given by Dr. Wells, 

 and, to me at least, it appears perfectly satisfactory and complete. 

 He employs it to explain a great number of facts, some of which 

 he had mentioned in enumerating the phenomena of dew, and 

 others are noticed for the first time here. I would with much plea- 

 sure run over this enumeration of facts, and the luminous explana- 

 tion of them given by Dr. Wells, which in most cases is perfectly 

 satisfactory ; but want of room obliges me to refer the reader to the 

 Essay itself: and I do this with the less reluctance, because most of 

 the phenomena explain themselves as it were spontaneously by the 

 mere application of the theory to them. 



The third part of this Essay treats of several appearances connected 

 wllh dew. By this, Dr. Wells means several appearances produced 

 by the same cause as dew, but generally ascribed to other causes. 

 These appearances are the following: — 



1. During winter, Dr. Wells observed the panes of the windows 

 of his bed-chamber moist on the inside; but those which had been 

 covered by an inside shutter during the night were much more so 

 than those that had been uncovered. He found upon examination 

 that the covered panes were 3° colder than the uncovered. '1 he 

 cause was this : — The shutter protected the panes from the heat 



