THE ANEROID BAROMETER 
107 
3. Any considerable blow is likely to throw the instru¬ 
ment out of order for the time being, if not permanently. 
Two instruments carried are a considerable insurance. 
4. The aneroid should always be held in the same posi¬ 
tion when read, and be given a little time to adjust itself. 
By gentle tapping on the face the observer should assure 
himself that its various parts are all free and in working 
order. 
5. In starting out for work it is well to carry the instru¬ 
ment a while, so as to get it into its regular field working 
order, before reading on the base station. 
6. One should check on points of known elevation as 
often as possible, and, if there is a choice of readings to 
refer to, he should depend on that which is nearer, time 
and elevation both considered. 
7. A general caution may be needed that the proper 
use of the instrument is to obtain relative elevation of 
points by means of readings on the two. One must not 
expect by one reading to obtain his height above sea 
level. 
Reduction of Aneroid Readings by Use of the 
Tables and with Correction for Temperature 
and Weather Changes 
(See tables on pages 111 and 112) 
Problem I. — Given barometric readings on two stations 
and temperature at each , to find the difference in elevation 
of the two points. 
Rule. — Enter the first column of Table T with the read¬ 
ings of the barometer on the two stations, and take out the 
corresponding numbers from column 2 (column 3 is for 
help in interpolating). Take the difference between these 
two figures. Call this result for the present a. 
Add the two temperatures together (or if the tempera¬ 
tures of the two stations do not differ materially, multiply 
that of the region by two). With this enter Table II, that 
for temperature correction, and find in column 1 the near¬ 
est number of degrees given. Take out of column 2 the 
number corresponding, noting the + or — sign, and 
