A higher degree of accuracy is obtained by using the multiplier obtained from the following table prepared by Mr. G. J. Symons:
| Mean temperature | 30° | 40° | 50° | 60° | 70° | 80° |
|---|---|---|---|---|---|---|
| Mean pressure, 27in. | 9.7 | 9.9 | 10.1 | 10.3 | 10.6 | 10.8 |
| ””28in. | 9.3 | 9.5 | 9.8 | 10.0 | 10.2 | 10.4 |
| ””29in. | 9.0 | 9.2 | 9.4 | 9.6 | 9.8 | 10.0 |
| ””30in. | 8.7 | 8.9 | 9.1 | 9.3 | 9.5 | 9.7 |
To find the difference in height between two stations: Find the mean pressure; also the mean temperature. The number in the table corresponding to these two means, if multiplied by the difference of the barometric pressures in hundredths of an inch, will give the difference in altitude very nearly.
In the absence of a table to aid in computation, but having an Aneroid with the scale of feet, use the formula,
adding
of the estimated altitude for every degree, the average temperature is above 55°, and subtracting a like amount when it is below. D, is the difference of altitude in feet; H and h are the readings in feet from the Aneroid scale. This gives fair approximations up to 3000 feet.
For accurate results use one of the following methods: Having Airy’s table ([Table 1]) and an Aneroid carefully graduated to inches; Take the reading in inches of the barometric scale at both lower and upper stations; also the temperature at both stations. Find from the table the heights in feet corresponding to the barometer readings. Subtract them and multiply the remainder by
