CORRECTION TABLE.
| For Division. | For Temperature. |
| 26.0″ = -0.02 | 28° to 48° = 0 |
| 26.5″ = -0.03 | 52° = +0.01 |
| 27.0″ = -0.03 | 56° = +0.015 |
| 27.5″ = -0.02 | 60° = +0.025 |
| 28.0″ = 0 | 64° = +0.035 |
| 28.5″ = +0.03 | 68° = +0.04 |
| 29.0″ = +0.06 | 72° = +0.05 |
| 29.5″ = +0.10 | 76° = +0.07 |
| 30.0″ = +0.14 | 80° = +0.09 |
| 30.5″ = +0.19 | 84° = +0.11 |
| 31.0″ = +0.25 | 92° = +0.15 |
The temperatures are, of course, taken from the thermometer that forms a part of the instrument, and which, when the latter is carried slung from the shoulder, may exhibit a temperature considerably higher than that of the air.
A smaller and ruder instrument called the Pocket Aneroid is made by the Zurich manufacturers. It is only 1½ inches in diameter and 1¼ inches high. A bar fastened to the top of the vacuum box takes the place of the lever in the larger instrument.
A larger size is also made in which the movements of the vacuum box are directly observed with a compound microscope.
Fig. 8.
Fig. 9.
There is no doubt that all aneroids need a careful comparison with standard instruments or a series of trials upon known altitudes, in order to determine the proper corrections. Such trials should be made at different temperatures and under different conditions as to rising or falling at the time of observation. The tables of corrections furnished by the maker cannot well be substituted for those made by a careful observer deduced from systematic work. The air pump, the hot chamber and the freezing box are convenient, but inadequate substitutes for a large number of trials under normal conditions.