-(52 - 32) × 30 × ·00014 = ·084,
making the true reading 30 - ·084 = 29·916 inches at 32° Fahr.
Tables for correction without any calculation will be found in Molesworth's and other pocket-books.
826.—Gravity Correction.—The force of gravity decreases as we ascend to a higher level in proportion to the square of the distance from the centre of the earth. It follows that the force of gravity as we ascend at the equator diminishes at a less rapid rate than at the poles. Its amount is always small—on an average it may be taken at about 0·001 inch of mercury per 400 feet of ascent.
Time.—Humboldt discovered that the barometer varied within the tropics at different hours of the day. This has also been found to be general to some extent in all countries, depending upon many conditions. It is only important for consideration of altitude measurements, that it is advisable if possible to take the upper and lower stations simultaneously by a pair of barometers for exact determination of altitude.
827.—Aneroid Barometer.—The first introduction of this instrument into England was by Pierre Armand, le Comte de Fontainmareau.[59] This instrument consisted of a vacuum chamber as its prime mover. The chamber was made a flat cylindrical box, with its upper surface of thin metal, with corrugations covering its surface in concentric rings. The chamber was filled with a number of spiral springs which resisted the pressure of air, to prevent the collapsing of the corrugated surface when the chamber was exhausted, and so placed the surface in equilibrium with the pressure it received from the atmosphere. The movements under various pressures were multiplied by gear work and levers so as to make a small movement of the corrugated surface evident in the extent of motion of an index hand reading upon a dial.
Fig. 393.—Stanley's civil engineer's aneroid.
Larger image
The aneroid practically in its present form was devised by Lucien Vidie from 1848 to 1862.[60] In this instrument the vacuum chamber, which is a thin, flat, circular box, is corrugated equally on both sides, so as to obtain double area of active surface under atmospheric pressure to that of the older form. The chamber has its surfaces drawn apart by an exterior spring, the point of communication or tension being placed at the centre of its corrugated sides only.