RADIATION OF HEAT. 



of a clear sky, received no considerable rays of heat to. supply the loss sus- 

 tained by their radiation. This loss in the metal was inconsiderable, and 

 therefore it maintained its temperature nearly or altogether equal to that of the 

 air ; the glass, however, radiating more abundantly, and absorbing little, suf- 

 fers a depression of temperature. The glass, therefore, presented a cold sur- 

 face to the air contiguous to it, and reduced the temperature of that air, until it 

 attained that temperature at which it was below a state of saturation with re-, 

 spect to the vapor with which it was charged ; a deposition of vapor, therefore, 

 took place on the glass. 



This discovery of Dufay remained a barren fact until the attention of Dr. 

 Wells was directed to the subject. The result of his inquiries was the dis- 

 covery of the cause of the phenomena of dew, and affords one of the most 

 beautiful instances of inductive reasoning which any part of the history of phys- 

 ical discovery has presented. Dr. Wells argued that, as a clear and cloudless 

 sky radiates little or no heat toward the surface of the earth, all objects placed 

 on the surface which are good radiators must necessarily fall in temperature 

 during the night, if they be in a situation in which they are not exposed to the 

 radiation of other objects in their neighborhood. Grass and other products of 

 vegetation are, in general, good radiators of heat. The vegetation which cov- 

 ers the surface of the ground in an open, champaign country, on a clear night, 

 will therefore undergo a depression of temperature, because it will absorb less 

 heat than it radiates. This fact was ascertained by direct experiment, both by 

 Dr. AVells and Mr. Six. A thermometer, laid on a grass plot on a cleai night, 

 was observed to sink even so much as 20 below another thermometer sus- 

 pended at some height above the ground. The vegetables, which thus acquire 

 a lower temperature than the atmosphere, reduce the .air immediately contigu- 

 ous to them to a temperature below saturation, and a proportionally copious 

 condensation of vapor takes place, and a deposition of dew is formed on the 

 leaves and flowers of all vegetables. In fact, every object, in proportion as it 

 is a good radiator, receives a deposition of moisture. On the other hand, ob- 

 jects which are bad radiators are observed to be free from it. Blades of grass 

 sustain large, pellucid dew-drops, while the naked soil in their neighborhood is 

 free from them. 



In the close and sheltered streets of cities the deposition of dew is very rare- 

 ly observed, because there the objects are necessarily exposed to each other's 

 radiation, and an interchange of heat takes place which maintains them at a 

 temperature uniform with that of the air. A deposition of dew, in this case, 

 can only take place when the natural temperature of the air falls below its point 

 of saturation. 



In an obscure, cloudy night no deposition of dew takes place, because in this 

 case, although the vegetable productions radiate heat as powerfully as before, 

 yet the clouds are also radiators, and they transmit heat, which, being absorbed 

 by the vegetables, their temperature is prevented from sinking much below that 

 of the atmosphere. 



