350 On Rain, Evaporation, Se. 
will not diminish, and probably not increase 
much.* 
It remains to estimate the quantity of dew 
that falls in'a year.—Some have doubted whether 
dew is derived from the air or the earth; but a 
proper attention to the phenomena will satisfy 
us, that it is a deposition of water, evaporated 
during the heat of the day, With respect to the 
quantity that falls in a year, we are much at a 
loss, as no daily observations have been made for 
a series of time that I know of: indeed, it would 
be difficult to prescribe a mode of observation. 
Dr. Halest relates some experiments made to 
determine the quantity of dew that falls upon 
moist earth, from which he estimates the annual 
dew at 3. 28 inches. But it is probable that the 
dew which is deposited on grass is much more 
copious than what falls on moist earth, because 
grass exposes much more surface in a given 
acre of ground. If we take the dew at 5 inches 
annually, it will probably not be much over- 
rated: supposing it should be over-rated, the 
* The editors of the Encyclopedia, under the article 
Weather, from 16 places of observation, make the an- 
nual mean for Great Britain 32. 53 inches ; and M. Cotte, 
in the Journal de Physique for 1791, gives a mean de- 
rived from 147 places in different parts of the world 
equal to 34. 7 inches, 
+ Veg. Statics. Vol. 1. Page 52, 
