store, is rendered quite damp; but it may after- 
wards become dry, on being transported to a 
warmer situation. Such is the case with the 
sea-breeze, particularly in summer. It arrives 
on the shore cold and moist; but as it advances 
into the interior of the continent, it grows milder 
and drier. The moisture deposited by a body of 
air in minute globules, which remain suspended 
or subside slowly in the atmosphere, constitutes 
a cloud. When it comes near us, whether it 
hovers on the tops of the hills or spreads over 
the valleys, it receives the name of a fog. The 
production of rain has, from the earliest times, 
engaged the attention of philosophers; but it 
was reserved for Dr. James Hutton, of Edin- 
burgh, to afford the true solution of the problem. 
His theory of rain was made known in 1787; 
since which period it has been greatly neglected 
by writers upon meteorology, until within a very 
few years. We shall now give an outline of Dr. 
Hutton’s views. Air, in cooling, it is known, 
has the property of depositing the moisture it 
contains. But how, it may be asked, is it cooled 
in the free atmosphere, unless by the contact or 
commixture of a colder portion of the same fluid ? 
Now the portion of air which is chilled must, in 
an equal degree, warm the other. If, in conse- 
quence of this mutual change of condition, the 
former be disposed to resign its moisture, the 
latter is more inclined to retain it; and, conse- 
quently, if such opposite effects were balanced, 
there could on the whole be no precipitation of 
moisture. The separation of moisture, on the 
mixing of two masses of damp air at different 
temperatures, would therefore prove, that the 
dissolving power of air suffers more diminution 
from losing part of the combined heat, than it 
acquires augmentation from gaining an equal 
measure of it; and, consequently, this power 
must, under equal accessions of heat, increase 
more slowly at first than it does afterwards, thus 
advancing always with accumulated celerity. 
The quantity of moisture which air can hold, 
thus increases in a much faster ratio than its 
temperature. This great principle in the eco- 
nomy of nature was traced by Dr. Hutton from 
indirect experience. It is the simplest of the 
accelerating kind, and perfectly agrees with the 
law of solution, which the hygrometer has esta- 
blished. Suppose equal bulks of air in a state 
of saturation, and at the different temperatures 
of 15 and 45 centesimal degrees, were inter- 
mixed; the compound arising from such union 
will evidently have the mean temperature of 30°. 
But since, at these temperatures, the one portion 
held 200 parts of humidity, and the other 800, 
the aggregate must contain 1,000 parts,.or either 
half of it, 500; at the mean or resulting tem- 
perature, however, this portion is only capable 
of suspending 400 parts of humidity, and, conse- 
quently, the difference, or 100 parts, amounting 
to the two hundredth part of the whole weight 
of air, must be precipitated from the compound 
——. $$$ “) | 
’ 
RAIN, — 
13 
mass. In this example, it has been assumed 
that the portions of differently heated air were 
saturated with moisture before mixing; but it 
is only required that they should approximate — 
to this condition. The effect, however, of simple 
commixture would, in most cases, be very small. 
To explain the actual phenomena, we must have 
recourse to the mutual operation of a chill and of 
a warm current driving swiftly in opposite direc- 
tions, and continually mixing and ‘shifting their 
surfaces. By this rapidity, a larger volume of 
the fluid is brought into contact in a given time. 
Suppose, for instance, the one current to have a 
temperature of 50°, and the other that of 70° 
Fahr.; the blending surfaces will therefore as- 
sume the mean temperature of 60°. Conse- 
quently the two streams throw together 200 and 
3342 parts of moisture, making 567:1 parts for 
the compound, which, at its actual temperature, 
can hold only 258°6 parts; the difference, or 8'6 
parts, forms the measure of precipitation, corre- 
sponding to the 2325th of the whole weight of the 
commixed air. It would thtis require a column 
of air 30 miles in length to furnish, over a given 
spot, and in the space of an hour, a deposit of 
moisture equal to the height of an inch. If the 
sum of the opposite velocities amounted to 60 
miles an hour, and the intermingling influence 
extended but to a quarter. of an inch at the 
grazing surfaces, there would still, on this sup- 
position, be produced, in the same time, a fall of 
rain reaching to half an inch in altitude. 
quantities come within the limits of probability, 
and agree sufficiently with experience and obser- | 
vation. But in the higher temperatures, though 
the difference of the heat between the opposite 
strata of air should remain the same, the measure 
of aqueous precipitation is greatly increased. 
Thus, while the mixture of equal masses of air, 
at the temperatures of 40° and 60°, is only 6°6, 
that from a like mixture of 80° and 100° amounts 
to 19. This result is entirely conformable to 
observation, for showers are most copious during 
hot weather and in the tropical climates. The 
quantity of rain precipitated from the atmo- 
sphere, thus depends upon a variety of circum- 
stances,—on the previous dampness of the com- 
mixed portions of the fluid,—their difference of 
heat,—the elevation of their mean temperature, 
and the extent of the combination which takes 
place. When the deposition is slow, the very 
minute aqueous globules remain suspended, and 
form clouds; but if it be rapid and copious, those 
particles conglomerate, and produce, according 
to the temperature of the medium through which 
they descend, rain, mist, snow, or hail. The 
foregoing theory tallies precisely with what we 
experience in the connexion of rain with the vari- 
able nature of the winds. Steady dry weather is 
always accompanied by a steady direction of the 
wind; whereas, in rainy weather, the winds are 
unsteady and variable. The heavy rains that 
fallin India always take place during the shift- 
These | 
