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 2$ 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 trom damage, also from extreme heat and 

 rapid changes of tempera turfc. 



