[Footnote 10: James Belville, Esq., Royal Observatory, Greenwich.] [Footnote 11: Sir Leopold McClintock.] [Illustration: 32. Aneroid Barometer. Full size.] By a suitable arrangement of clockwork, revolving a cylinder bearing prepared paper, the aneroid barometer forms an admirable self-recording instrument, showing at a glance the height of the barometer: whether it is falling or rising, for how long it has been doing so, and at what rate the change is taking place, whether at the rate of 1/10th per hour, or 1/10th in twenty-four hours—facts which can only be obtained by very frequent and regular observations from an ordinary barometer, but which are nevertheless essential to a reliable “weather forecast.”[12] [Footnote 12: _The Aneroid Barometer: How to Buy and How to Use it._ By a Fellow of the Meteorological Society. Post free for six stamps, from any bookseller or optician.] The height of mountains may also be determined by the temperature at which water boils, as this depends on the pressure of the atmosphere, and according to Deschanel, “just as we can determine the boiling-point of water when the external pressure is given, so if the boiling-point be known we can determine the external pressure,” and as this varies with the elevation above sea-level, the boiling-point of water also varies. These facts induced Wollaston to attempt the determination of heights of mountains by an apparatus which he called the Barometric Thermometer, subsequently modified by Regnault and called a Hypsometer, but now more generally known as a Boiling-point Thermometer. [Illustration: 33. Boiling-point Thermometer. Scale about 1/3.] A portable form of boiling-point thermometer is shown at Fig. 33, which is much used by Alpine travellers, and forms a trustworthy check on the aneroid and barometer. CONCISE TABLES FOR CALCULATING HEIGHTS BY MEANS OF BAROMETER OR ANEROID, AND ALSO BY THE BOILING-POINT THERMOMETER.