HOAH FEOST. 



exposed to each other's influence, and an interchange of heat by 

 radiation takes place so as to maintain their temperature ; besides 

 which, the objects found there are not as strong radiators as the 

 foliage and flowers of vegetables. 



60. When the cold which follows the condensation of vapour 

 falls below 32, what would otherwise be DEW becomes HOAR FEOST. 

 For the same reason that dew is deposited when the temperature 

 of the air is above the point of saturation, hoar frost may be 

 manifested when the temperature of the air is many degrees above 

 the point of congelation ; for in this case, as in that of dew, the 

 objects on which the hoar frost collects lose so much heat by their 

 strong radiation, that while the atmosphere may be above 40 they 

 will fall below 32. In such cases, a dew is first deposited upon 

 them which soon congeals, and forms the needles and crystals with 

 which every observer is familiar. 



The hoar frost is sparingly or not at all formed upon the naked 

 earth, or on stones or wood, while it is profusely collected on 

 leaves and flowers. The latter are strong, the former feeble 

 radiators. 



Glass is a good radiator. The panes of a window fall during 

 the night to a temperature below 32, although the air of the room 

 be at a much higher temperature. Condensation and a profuse 

 deposition of moisture takes place on their inner surfaces, which 

 soon congeals and exhibits the crystallised coating so often 

 witnessed. 



The frosts of spring and autumn, which so frequently are 

 attended with injury to the crops of the farmer and gardener, 

 proceed generally not from the congelation of moisture deposited 

 from the atmosphere, but from the congelation of their own proper 

 moisture by the radiation of their temperature caused by the 

 nocturnal radiation, which in other cases produces dew or hoar 

 frost. The young buds of leaves and flowers in spring, and the 

 grain and fruit in autumn, being reduced by radiation below 32, 

 while the atmosphere is many degrees above that temperature, 

 the water which forms part of their composition is frozen, and 

 blight ensues. 



These principles, which serve to explain the cause of the evil, 

 also suggest its remedy. It is only necessary to shelter the object 

 from exposure to the unclouded sky, which may be done by 

 matting, gauze, and various other expedients. 



66. In tropical climates the principle of nocturnal radiation has 

 supplied the means of the artificial production of ice. This process, 

 which is conducted on a considerable scale in Bengal, where some 

 establishments for the purpose employ several hundred men, 

 consists in placing water in shallow pans of ivnglazed pottery in a 



93 



