106 A MANUAL FOR NORTHERN WOODSMEN 
or a day at a time and still make fairly satisfactory height 
determinations. 
This is all good in theory, but it must be said that in 
practice it does not always work out to one’s entire sat¬ 
isfaction. The air, in the first place, is not the homoge¬ 
neous fluid that it has been considered, but varies more or 
less from point to point. Then aneroids are not sure in 
their workings. Different instruments of the same make 
and cost vary greatly in reliability, and the observer needs 
to watch the best of them to see that they do not get out 
of order or play some kind of a trick. Barographs, again, 
are not thoroughly reliable. In particular, some of them 
do not follow the changes in pressure as fast as the port¬ 
able instrument. Nevertheless, trial has shown that by 
the methods outlined sufficiently accurate results for many 
purposes can be obtained. In general it may be said of 
aneroid work that, while it cannot be counted on for re¬ 
fined accuracy, there is a large field open to it of good, 
useful work which no other instrument, on account of con¬ 
siderations of cost, can do. It is particularly serviceable in 
a timbered country where it is difficult to see from point to 
point, having there the same sort of advantage that the 
compass possesses in the same field. 
Aneroids for ordinary work should be 2j to 3 inches in 
diameter, graduated to the equivalent of 20 feet, and have 
as open a scale as may be. Such instruments cost from 
$20 to $35. For the finer class of work it may be advisable 
to employ a larger and more delicate instrument furnished 
with a vernier. A barograph costs from $40 to $50. Ther¬ 
mometers suitable for the work, in a nickel or rubber case 
about the size of a lead pencil, can be had for $.50 to $1 
each. 
The following Working Rules have grown out of the 
experience of the writer and others: 
1. Each instrument should be tested not only under 
the air pump but for general behavior in the field. 
2. The best place to carry an aneroid while at woods 
work is in a leather case hung on the belt. The case serves 
to protect it from damage, also from extreme heat and 
rapid changes of temperature. 
