THE SCIENCE OF A SNOW-FLAKE. 
47 
have the wilderness of waters called the sea. The ocean covers 
about three-fourths of the surface of the globe, or it occupies 
nearly 110,849,000 square British miles, and, equally over both 
sea and land, the vast atmospheric ocean flows. If we place per- 
fectly dry air, contained in a bell glass, over water, we shall find 
that water-vapour will rapidly ascend and diffuse itself through 
it — the quantity being regulated by the temperature of the 
apartment in which the experiment is made. At all tempera- 
tures, down to that at which water freezes, the air takes up 
water-vapour ; and this process is facilitated by the constant 
movements of the sea and the currents of the atmosphere. 
Therefore, we learn that the envelope surrounding this earth is 
an atmosphere of permanently elastic fluid, mixed with aqueous 
vapour in constantly varying proportions, the variations being- 
regulated by the temperature. Meteorologists have found it 
necessary to a clear understanding of the phenomena which are 
brought under their consideration, to study the conditions which 
would prevail, if the atmospheric elastic fluid — Air — existed in a 
perfectly dry state. The air does not derive much heat from 
the sun-rays passing through it, but it is warmed by its contact 
with the earth ; and this heat is conveyed from particle to par- 
ticle — this process being known as convection, the act of carry- 
ing or conveying. The solar rays fell with different degrees of 
intensity on the equatorial and polar regions ; we have in the 
former the maximum and in the latter the minimum of heat 
absorbed from the sun. As a particle of air becomes heated it 
expands, and becoming specifically lighter than the particles 
above it, it ascends, giving place to the colder and heavier 
ones. Thus the heating power of the sun becomes the main- 
spring of all the motions of the atmosphere, and, indeed, of those 
of the ocean. From the lands under the equator an upward 
current of air is thus generated, and the space occupied by the 
air thus removed, is supplied by currents of colder air flowing in 
from the poles. The aerial currents are complicated by the mo- 
tion of the Earth and by other conditions which cannot be consi- 
dered here. Sufficient for the present purpose that we under- 
stand that they are, in the main, the result of processes of heating 
and cooling which are regulated by an unvarying law. 
The habitudes of an atmosphere of pure a(][ueous vapour have 
also been, necessarily, the subject of close and attentive study; 
but we have to consider only the habitudes of a gaseous atmo- 
sphere mixed with vapour, that vapour being derived from the 
oceanic waters in the first instance, and from the evaporation, 
which is constantly going on over the land, of the waters 
which have fallen from the air. Dr. Dalton discovered that the 
evaporation of water has the same limits in air as in a vacuum. 
Hence it is only necessary to know the quantity of vapour which 
