Chap. 5.] Dew. 48 ^ 



When fogs rife to a confiderable height m the at- 

 mofphere, and are collected in a denje ftate, whether 

 this happens from any action of the air or from other 

 caufes, they 'form clouds, which float in different re- 

 gions of the atmofphere according to their fpecific 

 gravity j fince they neceflarily rife or fall, till they 

 arrive at that part of the atmofphere which is in 

 equilibrium with themfelves. As the atmofphere is 

 heavieft below, denfe and thick clouds, which are 

 t the point of uniting into rain, float near the furface 

 pf the earth, while the fleecy and thin clouds foar 

 aloft. We often obferve both kinds at different heights 

 in the atmofphere at the fame time. As clpuds are 

 formed of water, they are moft copioufly produced 

 where the air has moft opportunity of acting on that 

 fluid. Confequently winds which blow from the weft 

 and fouth weft, from the Atlantic ocean, bring more 

 clouds to this country than eafterly winds, which only 

 pafs over a narrow channel of the lea. 



During the day time the fun heats the earth, water, 

 air, and every thing which is expofed to its rays. The 

 heat communicated to all thefe bodies is diminiflied 

 after fun fet, but the air is more fuddenly cooled than 

 the more folid bodies. Heat, therefore, which has 

 always a difpofition to diftribute itfelf equally, pafies 

 from the furface of water and earth into the air, and 

 carries with it fome aqueous particles, which are fuf- 

 pended near the furface. When the cold increafes 

 during the night, thefe vapours are condenfed, and 

 occasion that dampnefs which is felt on the clothes of 

 perfons expofed to the night air, and form that dew 

 which is fo refrefhing to vegetables fcorched during the 

 " day time by the fummer's fun. 



In cold weather the dew becomes frozen into hoar 

 froft, for the formation of which it is not neceflary 



that 



