D E W 
grains5 and decreafed in weight hy the evaporation of 
the day, i ounce 282 grains : fo that 540 grains more 
are evaporated from the earth every twenty-four hours 
in fummer, than the dew that falls in the night ; i. e. in 
twenty-one days near twenty-fix ounces from a circular 
area of a foot diameter. Now, if 180 grains of dew, 
■falling in one night on fuch an area, which is equal to 
1 13 fquare inches, be equally fpread on the furface, its 
depth will be the 159th part of an inch. He likewife 
found that the depth of dew in a winter’s night was the 
ninetieth part of an inch. If, therefore, we allow 159 
nights for the extent of the fummer’s dew, it will in that 
time amount to one inch in depth ; and reckoning the 
remaining 206 nights for the extent of the winter’s dew, 
it will produce 2-28 inches depth ; and the dew of the 
whole year will amount to 3-28 inches depth. But 
the quantity which evaporated in a fair fummer’s day 
from the fame furface, being 1 ounce 282 grains, gives 
the fortieth part of an inch deep for evaporation, which 
is four times as much as fell at night. Dr. Hales ob- 
ferves, that the evaporation of a winter’s day is nearly 
the fame as in a fummer’s day ; 'the earth’s greater moif- 
ture in winter compenfating for the fun’s greater heat in 
fummer. Hales's Vegetable Statics, vol. i. p. 52 of 4th edit. 
Signor Beccaria made feveral experiments to demonfirate 
the exiftence of the electricity that is produced by dew. 
He obferves in general, that fuch electricity took place 
in clear and dry weather, during which no firong wind 
prevailed ; and that it depends on the quantity of the 
dew, as the eleCtricity of the rain depends on the quan¬ 
tity of the rain. He fometimes found that it began be¬ 
fore fun-let; at other times not till eleven o'clock at 
night. See his Artificial EleElf'icity .- Appendix, letter 3. 
The phenomena of the morning and evening dew have 
been lately explained in an ingenious manner, by C. A. 
Prieur, of the polyteclmical fchool. He remarks, that 
the moiflure deposited on bodies placed in the open air 
at the time of fun-fet, is not the fame as that feen after¬ 
wards on thefe bodies at the period of fun-rife : that 
-consequently there is an interruption in the phenome¬ 
non ; an evaporation of the dew or water which appears 
in the evening, and a new production of water in the 
morning. He had noticed alfo the common explanation 
in regard to the evening dew, viz, that the moifture feen 
in the evening ari'fes from the air being no longer able 
to retain the water which it held in folution during the 
day. But he enquires whence arif'es that wind which 
always proceeds from the quarter where the fun is, and 
which always accompanies this precipitation of water ? 
And, befides, how happens it, that when the fun is about 
to appear above the horizon, and even after it has been 
Somewhat heated by his prefence, there fhould be occa¬ 
sioned a greater cold, a Stronger wind, and a more abun¬ 
dant precipitation of water, than in the evening ? To 
Solve this difficulty, he has recourfe to thefe three fun¬ 
damental principles : 1. The air, every thing elfe being 
equal, dilfolves more water in proportion as it is denfer, 
that is to fay, more compreffed. 2. It dilfolves a greater 
quantity alio in proportion as its temperature is mere 
elevated. 3. The denfity or Specific gravity of air, which 
holds water in folution, is lefs than the Specific gravity 
of pure air, the pretfure and temperature being equal j 
and this Specific gravity is lefs according as the quantity 
of water dilfolved in the air is greater. It mull alfo be 
admitted, that the changes of p re flu re and weight which 
may take place in Some column's of the atmolphere, mult 
neceffarily difturb its equilibrium, and 'produce in it 
movements or currents. Thefe bafes being laid down, 
fuppofe for a moment that the earth, deprived of its ro¬ 
tary motion, is in an immoveable Rate before the fun ; and 
fet afide alfo the consideration of all local influence, or, 
in general, all caufes that tend to perturb the regularity 
of effects. In this Rate of things what will take place 
in our at.mofphere ? The air, expofed to the rays of the 
iun, w ill be heated, and particularly that part of it near 
7'83 
the earth, on account of its greater denfity and the re., 
verberation of the planet. The air thus heated will ac¬ 
quire a greater diffolving power, and w ill indeed diflblvc 
a great deal of water, if there be any in contatf with if. 
It will take up water, therefore, from the fea, from lakes, 
rivers, and ponds, and from all refervoirs, the furface ot 
which is expofed to its aClion : it will even abforb part 
of the moifiure with which the earth may be impregnated. 
Let us now confider what will take place in a vertical 
column aifumed in the atmofphere, and fir ft in one lying 
direCtly under the fun. The air being heated at its 
lower part, and becoming Specifically lighter by difiolv- 
ing water, will rife and be replaced by the adjacent air. 
A current, therefore, will be efiablifhed in the column 
from the bottom upwards. Were this column infulated 
from the neighbouring columns, as it would be if con¬ 
tained in a vertical tube, in proportion as the inferior air 
charged with water rofe, the upper air would defeend ; 
would become charged with water in its turn ; and would 
afeend, and be continually replaced without interruption. 
Befides, the air holding water in folution, when it at¬ 
tained to a certain height, and experiencing there a colder 
temperature with a lefs degree of preflure, would be¬ 
come fuper-faturated, and would abandon its ex»cefs of 
water. There would thus be formed a fog or cloud, 
which might continue to afeend in confequence ot the 
movement it had acquired, but which, when accumu¬ 
lated to a certain point, would fall down in rain. It 
may readily be conceived that this tranflation of air from 
a lower to a higher, and from a higher to a lower, fitua- 
tion, would be performed by reciprocal infiltration, or 
by a current in both directions regularly maintained. 
But things are not altogether fo in fuch a vertical co¬ 
lumn, becaufe in reality it is not Separated from the 
red. The latter is Subject to the fame effeCts, only that 
they are lefs in proportion as thefe columns are removed 
from the direction of the fun : the heat, folution of wa¬ 
ter, and afceniional force, go on decreafing the further 
they are diRant from that direction. If the furface of 
the earth, therefore, be fuppofed a plane, it will be ne- 
celiary to reprefent the air, which riles as a fort of cone, 
with its Summit directed towards the heating luminary ; 
and on the other hand, as the abfolute gravity of each' 
column is increafed by the whole of the water it dilfolves, 
the equilibrium requires that there Rtould be a dilcharge 
from each, into the lateral oneS; a circumRance which 
muR evidently be effected where there is the lead pref- 
fure. Thus, on the fame fuppofition of the earth being 
a plane, we fiiould lee the Superior air defeend and force 
itfelf along the (ides of the cone above-mentioned, pro¬ 
ducing, by this oblique motion at the furface ol the earth, 
a current proceeding in every direction from the quarter 
of the fun ; and this current would be increafed by the 
vacuum formed at thole places where the air, fufficiently 
elevated and cooled,’ precipitated its Superabundant wa¬ 
ter. We need only to modify this image a very little to 
make it correfpond with the contour of our globe. T he 
conical furface will be converted into a fpherical Segment 
inclofmg the enlightened portion of the earth, and ac¬ 
cording to the curvature of which, the before-mentioned 
currents will be eRabliihed. 
Such, then, will be the refult of the circumflances fup¬ 
pofed : water dilfolved by the inferior air in the part ex- 
poled to the fun; an afcending movement of the air in 
that part; defeending currents diverging from all lides, 
and prolonging themlelves over the earth ; thefe currents 
carrying with them water which they have taken from 
the heated columns that rife, and which are forced off' 
in a lateral direction ; this water thus carried away pre¬ 
cipitated, either becaufe the air, coming from the upper 
regions, is too cold to keep it dilfolved ; or on account 
of the diminution of preffure arifing from the weight of 
the columns being lels in proportion to their diRance 
from that immediately under the fun ; or in confequence 
of the mixture of currents with the atmofphcric airy 
. w hich 
