Celsius (of Upsala, 1742) used a scale of 100° between these points. Hence it is usually called Centigrade. This is the scale of international physical and chemical reports; and is generally used in laboratory-work. The maximum density of water is at 4° Centigrade = 39·2° F. The Fahrenheit scale is generally used in English-speaking countries. It is convenient for meteorological purposes as there is rarely any need to use ‘minus’ degrees for winter temperatures as in the Centigrade and Réaumur scales. For medical purposes it is also more convenient to have the normal body-temperature at 98·4° F. (close to 100°), than at 36·9° C. or at 29·5° R.[43] Footnote 43: Normal body-temperature is taken in France as 37° C. In Germany it is taken as 29·3° Réaumur = 97·9° F. To convert Centigrade degrees into Fahrenheit: Double the degrees; deduct 1/10; add 32°. E.g., 20° C.; 20 × 2 = 40; 40 - 4 = 36; 36 + 32 = 68° F. To convert Fahrenheit degrees into Centigrade: Deduct 32°; halve the degrees; add 1/9 (roughly 1/10 or a little more). E.g., 100° F.; - 32 = 68; 1/2 68 = 34; 34 + 3·4 = 37·4 (37·75 correct). 2. MEASURES OF DENSITY Specific gravity and the density of fluids at different temperatures were known in very ancient times. The ‘Eureka’ experiments of Archimedes are well known. Al-Khazini (1121)[44] determined the specific gravity of metals and of fluids at different temperatures as accurately as modern physicists have done. Footnote 44: _Book of the Balance of Wisdom_ (H. Carrington Bolton). The hydrometer, divided into 24 qiráts, was well known in the East, even before the time of the Caliphates. The density or specific gravity of solids and fluids is usually referred to that of distilled water at 62° F. = 16·6° C. It is sometimes referred to 4° C. = 39·2° F. There is no advantage in this temperature, that of water at its maximum density; corrections for temperature will always have to be made in exact determinations, while, in the approximate determinations of trade, the indoor standard of 62° F. requires no correction. The specific gravity of gases being in the ratio of their molecular weight, which is referred to that of hydrogen, this gas is taken as the standard. The density of solutions of salts, sugar, acids, &c., is referred to that of water or stated on a trade-scale usually indicating the percentage in solution. The density of spirits is referred to that of water, but their alcoholic strength to: (_a_) A scale indicating the volume of alcohol per cent. (Gay-Lussac and Tralles). (_b_) An arbitrary scale (Cartier and Baumé). (_c_) An excise-scale on the basis of proof-spirit (Sykes).