23 1 ASTRONOMY. 



when the direct influence of the sun. is removed in the evening, and the 

 surface of the earth thus no longer continues to acquire heat, at that 

 instant, from the ceaseless activity of heat to obtain an equilibrium, the 

 surface of the earth being the warmer body, radiates a portion of its 

 superfluous temperature into the surrounding space; and thus the air, 

 immediately in contact with the surface, becomes cooled below the point 

 of saturation, and gives off a portion of its water in the form of dew. It is 

 known, that the radiating power of bodies differ exceedingly according to 

 their composition, the nature of their surface, their colour, &c. These 

 differences, of course, produce corresponding effects on the deposition of 

 dew ; and, as beautifully demonstrated by Dr. Wells, explains its greater 

 or less deposition under certain circumstances, or its entire absence under 

 others. Thus, what formerly appeared so extraordinary, viz. why, in the 

 self-same state of the atmosphere, Sec. one portion of herbage should be 

 covered with dew, while another in the immediate neighbourhood should 

 remain dry, is no longer a mystery ; but is perfectly explicable, on the 

 supposition of their different radiating powers. The deposition of dew is 

 always most abundant during calm and cloudless nights, and in situations 

 freely exposed to the atmosphere. Whatever interferes in any way with 

 the process of radiation, as might be expected, has a great effect on the 

 deposition of dew. Hence the radiation of heat, and consequently the 

 deposition of dew, are obviated, not only by the slighted covering or shelter, 

 as by thin matting, or even muslin; by the neighbourhood of buildings, 

 and innumerable other impediments near the earth's surface ; but matters 

 interposed at a great distance from the earth's surface have precisely the 

 same effect. Thus clouds effectually prevent the radiation of heat from 

 the earth's surface ; so that cloudy nights are always warmer than those 

 which are clear, and in consequence there is usually on such nights 

 little or no deposition of dew. From dew there is an insensible transition 

 to hoar frost; hoar frost being, in fact, only frozen dew, and indicative of 

 greater cold. We observe, therefore, that frosty nights, like simply dewy 

 nights, are generally still and clear. The influence of radiation in produ- 

 cing cold at the earth's surface would scarcely be believed by inattentive 

 observers. Often on a calm night the temperature of a grass plot is 20 

 or 15 degress less than that of the air a few feet above it. Hence, as Mr. 

 Daniell has remarked, vegetables in our climate are, during ten months 

 of the year, liable to be exposed at night to a freezing temperature ; and, 

 even in July and August, to a temperature only two or three degrees 

 warmer. 



