cury, and to air in the tube. These errors may be corrected by comparison with a standard instrument.
The form of barometer known as the aneroid is also frequently used for the determination of heights, a graduated scale being added for this purpose. This scale is graduated by means of one of the barometric formulæ already referred to. The aneroid barometer usually consists of a metallic box from which the air has been exhausted, and differences of atmospheric pressure are recorded by a system of levers which act on an index hand which marks the reading on a graduated scale. In some forms of aneroid the box is not completely exhausted of air, and these are called "compensated aneroids," but the name is misleading, some of these instruments being more sensitive to changes of temperature than those not compensated. The aneroid is a very handy instrument and easily used, but for the purpose of measuring heights it is much inferior to the mercurial barometer. In some instruments the altitude scale is fixed at a certain reading, say thirty or thirty-one inches, and in others it is movable, and can be adjusted to any reading required. The latter seems the most convenient plan. In either case it is clear that absolute elevations above the sea level can not be determined with this instrument with any approach to accuracy, as there is no way of making the necessary corrections for variations in pressure, temperature, etc. The aneroid barometer should, therefore, be used only for finding differences of elevation, and for this purpose it will give fairly good approximate results in cases where extreme accuracy is not required.
To show the degree of accuracy attainable by the barometric method, two examples may be cited. From readings of a mercurial barometer at the summit of Mont Blanc and at the Geneva Observatory made by MM. Bravais and Martins in the year 1844, the height of the mountain above the level of the sea was computed to be 4,815·9 metres, or 15,800'44 feet. Corabeuf found by trigonometrical measurement a height of 15,783 feet, or 17'44 feet less than that indicated by the barometer.
The height of Mount Washington, in the United States, was found by a spirit level to be 6,293 feet above sea level, while the barometric method gave 6,291·7 feet, a close approximation. In some other cases, however, much larger differences have been found, and the good agreements quoted above may perhaps be considered as accidental.—Gentleman's Magazine.
