Chap.5] Valour ccndenjed on cold Surfaces. 13 1 



manner, and this moifcure in both cafes is produced 

 by the cold body (the glafs of water, the marble, or 

 the wall) condenfing the vapour with which the air is 

 charged from the breath and perfpiration of the com- 

 pany. Similar to this is the dew on the infide of win- 

 dows, which in cold weather is frequently frozen, and 

 affumes the forms of leaves, of trees, and of the moil 

 beautiful mofl~es 



Water cannot be expofed in open vefiHs without 

 fuffering a diminution of its bulk, and indeed in courfe 

 of time the whole will be exhaled. The vapour how- 

 ever which is thus formed is not fufficiently elaftic to 

 produce any of the common effects of vapour; for 

 water will remain in bottles corked up without forcing 

 the corks ; the vapour ftagnating over the furface of 

 the water, prevents a frefh quantity from riling : in- 

 deed the mere force of gravitation would in general 

 be fufficient to counteract the force of this fpontaneous 

 evaporation, was it not that the wind carries off the 

 quantity which h exhaled, which would otherwife be 

 fufpended and ftagnate over the fluid. This vapour, 

 it is to be obferved, proceeds always from the furface 

 of bodies ; and the greater the extent of furface, the 

 more copious the exhalation : it is obferved, therefore, 

 to rife more copioufly from a graffy plain, from the 

 pores of a fpunge, or from loofe earth, than from any 

 fingle furface; and it is always more or lefs in quantity, 

 according to the temperature of the atmofphere. 



The quantity of vapour which is raifcd in this man- 

 ner from the earth, has been efti mated by a very fimple 

 yet apparently accurate experiment of the Bifliop of 

 Landaff. Having provided a large drinking glafs, the 

 area of the mouth of which was twenty fquare inches, 

 he placed it with its mouth downwards on a grafs plat 

 which was mown clofe. The fun fhone bright and 

 K 4 hot, 



