The National tribune., 1891-01-01
THE WEATHER.
Some Facts About Measuring the Pressure OF the
Atmosphere.
By LIEUT.. J. p. FINLEY. U. 5. SIGNAL SERVICE.

By LIEUT.. J. p. FINLEY, U. s. SIGNAL SERVICE.
Correspondence invited. suggestions desired,
and information ehecrfully furnished. Every eE
fort employed to make this Bureau Of practical
To the readers of THE NATIONAL TRIBUNE. All
communications should be addres5ed-WedtheT,
The NATIoSAL TEfnCNE Office, Washington, ID. q.
Much confused and erroneous reasoning has
arisen from persons failing to distinguish be-
tween the pressure Of the atmosphere and its
ueigL. The barometer measures its pressure,
and this is equal TO its freight only when the air
is at perfect rest and undergoing no change Of
temperature; a condition which, of course, is
never completely fulfilled. Probably, on an
average, the actual weight Of the superin-
cumbent atmosphere is most nearly shown by
its pressure about or p. IN. and between
and 4 a. m. on any day. But the justice Of
tion OF tho diurnal oscillation of pressure and
it may be said truly that we know nothing
accurately Of the weight of the atmosphere,
but. only that it cannot DO very different from
the average pressure.
IL is best. in speaking OF the barometric con-
dition Of the atmosphere, 10 avoid the use Of
the term weight altogether unless there be
special reasons for referring TO it.
The barometer, although it has many forms
consists essentially of vessel Of mercury,
termed the cistern, in which is inverted glass
tube, closed at the top and opened below, and
which, before being inverted, has been entirely
filled with mercury. In virtue Of the law Of
fluid pressure, the air pressing on the mercury
Of the cistern can support in the tube a column
Of mercury, which presses on its base, with
a pressure equal to that Of the air on the
same area Of cistern surface. An air column,
resting on the surface of the sea- presses equally
with a column Of mercury Of the same dame
ter aud about 30 inches high, on the average.
IT, therefore, the tube be shorter than this
length, the barometer being at the sea level, it
will remain full of mercury. But il longer,
and n barometer is always longer than 80
inches, the mercury will fall to about that
hight, leaving a vacuum in the upper part of
the tube, which is known as the Torricellian
vacuum.
As tho pressure Of the air varies, so does
the hight of the baTometTic column, and the
greater the pressure Of tho atmosphere the
higher the column it supports in the tube.
Galileo first taught that the air has weight.
His pupil, TOrricelli, brilliantly demonstrated
this truth by showing that the weight Of the
air will uphold column of water, ard in the
course Of his fist series of observations the
occurrence of variations in the weight Of the
| air become apparent. Subsequently it was
seen that the air is an elastic fluid; that its
vertical weight is transmitted in all directions
| by il as an elastic pressure; and that the
barometer measures an elastic pressure by
means of the vertical pressure due to the
| weight Of a vertical column OF liquid. The
pressure of the air will not only uphold liquid
column, but will compress elastic bodies, in
Guiding the air itself Upon this pressure
| depends the refractIve index Of the air, which
varies with its density the boiling points OF
| liquids, and other phenomena. Upon these and
| other properties various forms Of the barom-
eter have been based. The mercurial baron
eter is adopted as the standard, while other
liquid baTometers and barometers depending
| upon the compression of elastic bodies, and
upon other properties, are used for diferential
| purposes.