116 Forest Club Annual 



or from elevations of previous camps. The data which was 

 used in determining the elevations of reconnaissance camps No. 

 3 and No. 4, as given below, illustrates these two methods. 



Camp No. 3 was located about five miles from a Geological 

 Survey bench mark which was 2808 feet above sea level. After 

 finishing the day's work in the field, one of the crews took two 

 aneroids to the bench mark and obtained readings at 8 : 24 o'clock 

 that evening which gave an average of 2855. The correction 

 curve, made from hourly readings of that day showed, that if the 

 aneroids could have been transferred from the bench mark to 

 the camp without the loss of more than a few minutes they 

 would have dropped from 2855 to 1515, thus showing a differ- 

 ence in elevation of 1340 feet between the two points. The ele- 

 vation of the bench mark was 2808, that of the camp was 1340 

 feet lower, or 1468 feet above sea level. 



The next morning the crew returned to camp and the 

 aneroids were again read, giving an average of 1430 at seven 

 o'clock. A second computation was made, vising this figure and 

 the one obtained the night before. If no correction were neces- 

 sary for the variation in atmospheric pressure the elevation of 

 the camp would have been 1425 feet lower than the bench mark 

 (2855, the reading at the bench mark at 8:24 in the evening, 

 minus 1430, the reading in camp the next morning at seven 

 o'clock, gives a difference of 1425 feet), or it would be 1373 

 feet above sea level. It was necessary, however, to apply a 

 positive correction, since the curve of the previous day showed 

 that the reading made at seven o'clock in the morning was 85 

 feet lower than that at 8 : 24 the previous evening in camp. 

 Applying the positive correction, that is, adding 85 to 1373, the 

 computed elevation of the camp, which assumes that ro atmos- 

 pheric variation is considered, was accepted as 1458 feet from 

 this set of figures. The average of these two methods gave 

 1463 feet. Another series of similar computations gave 1454 

 feet as an average. The mean of all these computations gave 

 1458 feet as the true elevation of the camp No. 3. 



It was customary in establishing a new bench mark, to make 

 three sets of observations when working from a Geological 

 Survey bench mark, and at least five sets of readings when 

 working from a camp elevation. As many more readings were 

 taken as couM be conveniently obtained, depending upon the 

 distance between the two points. It was assumed that the United 

 States Geological Survey bench marks were much more accurate 

 t^an those established by the reconnaissance crew. The data 

 givei in the folowing table was obtaine r1 while moving from 

 reconnaissance camp No. 3 to camp No. 4. In addition to the 



