364 PLEASANT WAYS IN SCIENCE. 



in the pride of half-knowledge," says Wells, " smiled at the 

 means frequently employed by gardeners to protect tender 

 plants from cold, as it appeared to me impossible that a thin 

 mat, or any such flimsy substance, could prevent them from 

 attaining the temperature of the atmosphere, by which alone 

 I thought them liable tobe injured. But when I had seen 

 that bodies on the surface of the earth become, during a still 

 and serene night, colder than the atmosphere, by radiating 

 their heat to the heavens, I perceived immediately a just 

 reason for the practice which I had before deemed useless." 



And now all the facts which had before seemed obscure 

 were accounted for. It had been noticed that metallic plates 

 were often dry when grass or wood was copiously moistened. 

 Now, we know that metals part unwillingly with their heat 

 by radiation, and therefore the temperature of a metal plate 

 exposed in the open air is considerably higher than that of 

 a neighbouring piece of wood. For a similar reason, de\v is 

 more freely deposited on grass than on gravel. Glass, again, 

 is a good radiator, so that dew is freely deposited on glass 

 objects, a circumstance which is very annoying to the teles- 

 copisL The remedy employed is founded on Wells' obser- 

 vations a cylinder of tin or card, called a dew-cap, is made 

 to project beyond the glass, and thus to act as a screen, and 

 prevent radiation. 



We can now also interpret the effects of a clear sky. 

 Clouds act the part of screens, and check the emission of 

 radiant heat from the earth. This fact has been noticed 

 before, but misinterpreted, by Gilbert White of Selborne. 

 "I have often observed," he says, " that cold seems to descend 

 from above ; for when a thermometer hangs abroad on a 

 frosty night, the intervention of a cloud shall immediately raise 

 the mercury ten degrees, and a clear sky shall again compel 

 it to descend to its former gauge." Another singular mis- 

 take had been made with reference to the power which clouds 

 possess of checking the emission of radiant heat It had 

 been observed that on moonlit nights the eyes are apt to 

 suffer in a peculiar way, which has occasionally brought on 



