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.
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.