==========+=========+==========
Feet above|Barometer| Relative
Sea Level | Inches |Capacities
----------+---------+----------
0 | 30.00 | 1.000
500 | 29.42 | .983
1000 | 28.87 | .967
1500 | 28.33 | .954
2000 | 27.79 | .938
2500 | 27.27 | .924
3000 | 26.76 | .909
3500 | 26.25 | .894
4000 | 25.75 | .879
4500 | 25.26 | .867
5000 | 24.78 | .856
6000 | 23.85 | .827
7000 | 22.95 | .800
8000 | 22.10 | .772
9000 | 21.22 | .750
10000 | 20.43 | .725
12000 | 18.92 | .677
----------+---------+----------
The fact that the heating effect of compressing air from an initial
pressure of 10 pounds absolute to 90 pounds gauge pressure is
theoretically equivalent to that of compression to 132 pounds at sea
level, _makes a two-stage arrangement more imperative in high level
work than under ordinary conditions_.
[Illustration: FIG. 373.]