50 PHYSICS. 
equal height with that surrounding it; suck some of the air from the tube, 
and additional water will enter, because the equilibrium of atmospheric 
pressure is disturbed. The air within becomes rarer and lighter; the 
external atmosphere, therefore, pressing upon the external surface of 
the water, forces it up into the tube until the air therein contained is com- 
pressed sufficiently to exert the same pressure with the outer, or, in other 
words, until the weight of the water raised is equal to the excess of external 
pressure. Exhaust the air entirely from the imside of the tube, and the 
water must rise until the weight of the column raised is equal to the weight 
of a column of air having the same base, and a height equal to that of the 
atmosphere. It has been found that a column of about 33 feet is the maxi- 
mum that can be raised in this manner. Torricelli from these facts 
established the following conclusion: for two different columns of fluids to be 
in equilibrium, they must be to each other inversely as their densities. 
Mercury is fourteen times heavier than water ; if, now, the pressure of the 
atmosphere sustain a column of water 33 feet in height, it will sustain one 
of mercury 22 feet. or about 29 inches. That this is actually the case is 
shown by a simple apparatus for measuring the pressure of the air, termed 
the Barometer, consisting essentially of a glass tube about 31 inches long, 
closed at one end and filled with mercury. After filling this tube, hold the 
finger on the open end, and inverting it in a basin of mercury, remove the 
finger. The height of the mercurial column remaining in the tube, which 
in places at a slight elevation above the sea amounts to a mean height of 
about 28.6 inches, serves as a measure of the pressure of the air, as this, act- 
ing on the external surface of the mercury in the basin, sustains that in the 
tube. Along the top of the mercurial column, a scale divided into inches 
and fractions of an inch is attached, sometimes on metal, sometimes on 
paper, and occasionally upon the tube itself. To ascertain the amount of 
atmospheric pressure upon any given surface, calculate the weight of 
a column of mercury whose base is that of the given surface, and whose 
height is that of the mercury in the barometer. 
Many different constructions of the barometer have been made. princi- 
pally reducible, however, to two kinds, cistern and syphon barometers. The 
common barometer (pl. 18, fig. 32) is one of the first kind. It consists of 
a long tube, B, curved beneath and dipping into the vessel or cistern, C, 
upon which the pressure of the external air can act, as it isopen. The 
whole is fastened to a board, A, and a scale, D, with a movable index, E, 
attached, to mark the variations of pressure by the rise or fall of the mer- 
cury. This scale is generally divided into inches, and tenths or twelfths, 
and avernier frequently attached to the index for measuring very slight 
variations. The small scale, F, serves to measure the mercury in the vessel 
or cistern. Attention must always be directed to the vertex of the con- 
vexity of the mercury, which is formed in the ascent. In filling the 
barometer, care must be taken that there are no bubbles of air in the mer- 
cury, or attached to the tube, these being driven out by boiling the mercury 
in the tube. If these are not expelled they will rise into the top of the tube, 
and exert atmospheric pressure upon the top of the mercurial column, thus 
224 
