ON CHEMISTRY. 
291 
with precision, to what altitude the atmosphere extends; but admit¬ 
ting that its limits may be forty-five or fifty miles above the surface- 
level of the sea, the pressure at that height, must be reduced below 
the power of common estimation : this may be somewhat elucidated 
by the following facts. At the surface of the ground,—water (as 
seen in the action of the common pump) may be raised by the pres¬ 
sure of the whole atmospheric column, nearly 34 feet. Mercury—a 
dense fluid metal,—may be made to ascend above £0 inches; but at 
the height of a few thousands of yards, a balloon of varnished, thin 
silk, filled with hydrogen gas (which is the lightest of all known 
fluids,—100 cubical inches weighing little more than two grains and 
a half,)—can no longer be supported. It cannot therefore be tin- 
philosophical to conjecture that, the bounds of the atmosphere are 
limited,—that they extend not many miles above the surface of the 
globe. 
I do not attempt to write a treatise on pneumatics; I merely wish 
to adduce a few plain facts, to prove, first that pressure and elasticitv 
exist; and second, to introduce the reader to an enquiry into the 
cause or source of the several phenomena which they exhibit. 
The Barometer, or weather-glass is an instrument by which the 
weight or pressure of the incumbent atmosphere is ascertained: its 
name is derived from two greek words pronounced bares and me- 
troii; the former implies iveight, the latter measure : the instrument 
measures the weight of the air, in inches and parts of inches. 
The general average height of the quicksilver in the weather-glass 
throughout Britain, may be somewhat under 30 inches; or in other 
words, the atmosphere,—which at a medium pressure at the earth’s 
surface, exerts a force equal to the Aveight of fifteen pounds upon 
every square inch of surface,—is, under ordinary circumstances, able 
to sustain a column of mercury at the height of nearly 30 inches in 
the tube of the instrument. But variations in atmospheric pressure 
are continually taking place, and the rise or fall of the Barometer 
indicates these changes; it does not however point to their causes ; 
these remain involved in mystery. It has been supposed that, “ the 
increase of weight proceeds from the quantity of water dissolved in 
the air; this notion is however, refuted by the simple fact that, when 
the barometer stands highest, the air is most dry" —and vice versa. 
Again — 1 “ the order of the phenomena corresponds with the facts, 
that the barometer is most steady when the weather is clear, and 
fluctuates most with clouds and rain." 
These phenomena have occurred, and been noticed, and will be 
so again and again ; but the very converse ol them all have occur- 
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