82 THE HABITAT 
120. The barometer. To secure convenience and accuracy in the de- 
termination of altitude, it is necessary to use both a mercurial and an 
aneroid barometer. The latter is by far the most serviceable for. field work, 
but it requires frequent standardizing by means of the former. The mer- 
curial form is much more accurate and.should be read daily in the base 
station. It is practically impossible to carry it in the field, except in the 
so-called mountain form, which is of great service in establishing the alti- 
tudes of a series of stations. In use the aneroid barometer may be checked 
daily by the mercurial standard, or it may be set at the altitude of the base 
station, thus giving a direct reading. After the normal pressure at the 
base has once been ascertained, however, the most satisfactory method is 
to set the aneroid each day by the standard, at the same time noting the 
pressure deviation in feet of elevation (see 
p. 46). The absolute elevation of the var- 
ious stations of a series may be determined 
either by adding or subtracting this devia- 
tion from the actual reading at the station, 
or by noting the change from the base 
station, and then adding or subtracting this 
from the normal of the latter. When it is 
impossible to check the aneroid by means 
of a mercurial barometer, the average of a 
series of readings made at different days at 
one station, especially if taken during 
settled weather, will practically eliminate 
the daily fluctuations, and yield a result 
essentially accurate. Even in this event, 
the accuracy of the aneroid should be 
checked as often as possible, since the 
mechanism may go wrong at any time. The 
barograph, while a valuable instrument for base stations, is not at all neces- 
sary. These instruments can be obtained from all makers of meteorological 
apparatus, such as H. J. Green, and J. P. Friez. Anerdid barometers reading 
to 16,000 feet cost about $20; the price of the Richards aneroid barograph is 
$45. Ordinary observatory barometers cost $30-$40; the standard instru- 
ment sells at $75-$100. The mountain barometer, which is altogether the 
most serviceable for the ecologist, ranges from $30-$55, depending upon 
accessories, etc. 
SS x 
\y 
i <r 
cit 
spet 
OL 
3 
oo 
ron 
a 
<x 
S 
cS 
Fig. 26. Aneroid barometer. 
