153 



separates it into such small portions, that the air is more 

 ponderous than the steam, and of consequence remains 

 nearer the earth by its superior gravitation. 



2. The dew which usually falls inEnglamt in a year, 

 amounts to something more than three inches and a 

 quarter depth. The evaporation of a winter's day is 

 nearly the same as that of a summer's day. For the' 

 earth being moister in winter, that excess of moisture 

 answers to the excess of heat in summer. 



Within the tropics they have no rain for many months 

 together. But the dews are far greater than with us. 

 Yet the moisture evaporated in a summer's day, far ex- 

 ceeds that which falls in the night. Hence the dews 

 there cannot be of any benefit to the roots of the trees, 

 because they are remanded back from the earth by tiie 

 following day's heat, before they can soak to any con- 

 siderable depth. The great benefit therefore of dew 

 in hot weather, must be by being imbibed into vegeta- 

 bles, to refresh them for the present, and supply them 

 with moisture towards the expence of the succeeding day. 



Meantime the sun draws fresh supplies of moisture 

 from the strata of the earth, which by means of its pene- 

 trating warmth insinuates itself into the roots. By the 

 same genial heat it is carried up through their bodies 

 and branches, and thence passing into the leaves, it js 

 vigorously acted upon in those thin plates, till perspiring 

 through their surface, it mounts with rapidity in the 

 free air. 



But the strangest circumstance relating to dew, is 

 this. In the same night place several substances in the 

 open air, whilst a large dew falls : and some of them 

 will receive much of it, some little, and others none at 

 all. The drops make a sort of choice what bodies they 

 shall fix themselves to. Glass and crystals they fix 

 themselves to readily, and in the largest quantities. Me- 

 tals do not receive them at all, nor do the drops ever fix 

 on them. If a glass vessel be set out in the evening, on 

 a silver plate, the glass will be found quite covered with 

 <lew, and the silver perfectly dry. China-ware is a sort of 

 H 



