Meteorological Sketches. 51 
or envelope, which in the immediate vicinity of the earth, is greatly 
compressed by its own weight, and which in its most expanded and 
tenuous state is supposed to extend itself to the height of only for- 
ty-five or fifty miles from the earth’s surface. Its superincumbent 
pressure or weight is ascertained by means of the barometer, and is 
equal to a column of mercury about thirty inches in height. By 
means of this instrument we learn that one half its weight or actual 
quantity is within three miles and a half of the surface of the ocean ; 
and it is within this limit that nearly all the visible or important phe- 
nomena of the atmosphere are apparently developed. The super- 
ficial area of the lower surface of the atmosphere is equal to about 
200,000,000 square miles ; and as a compression of the whole mass 
to the common density which it exhibits at the sea level, would re- 
duce its entire height to about five miles, it follows that by this stan- 
dard of comparison the height or thickness of the atmosphere is to 
its superficial extent in the proportion of only 1 to 40,000,000. 
These several facts are too important to be lost sight of, in our 
general reasonings upon the phenomena of the atmosphere; and the 
more so, as we are prone to give too much altitude to our concep- 
tions on these subjects. If we even consider the proper height or 
thickness of the atmosphere as equal to fifty miles, still, as compared 
with its entire surface, this is only equal to one five hundredth of the 
proportion which the thickness of a common sheet of paper, of the 
foolscap size, bears to its surface dimensions; and if we view the 
atmosphere either as condensed to the mean of the surface pressure, 
or in relation to the actual limit of all its tangible phenomena, it will 
only be equal to one five thousandth part of the proportional thick- 
ness here mentioned. We may hence perceive the inapplicability 
of analogical reasonings that are founded on the movements which 
occur in a chimney, or in an inclosed apartment, as attempted to be 
applied in explanation of the general movements of the atmosphere. 
Two instruments of modern invention, the barometer and ther- 
mometer, are truly invaluable as testing the condition of the atmos- 
phere, and their use should be familiar to every navigator. By the 
first, as we bave seen, the amount or weight of the superincumbent 
atmosphere, at any place, may always be accurately known, and by 
the indications of the other the temperature of the air as well as of 
the ocean, may be ascertained with equal precision. 
Among the most striking peculiarities of the atmosphere, are its 
rapid and almost constant movements of progression or circulation, 
