Men and Measures

1. Measures of Heat

Thermometric Scales

About 1595 Galileo made a thermometer, probably one with an air-bulb in which expansion of the air forces water down the tube.

Isaac Newton made an oil-thermometer with a scale of 12° between freezing-point and body-heat.

Fahrenheit, about 1714, made a mercurial thermometer, its 0 at the cold produced by a refrigerating mixture and 24° at body-heat. On this scale, freezing-point was 8° and boiling-point 53°. The quarter-degrees were then made whole degrees, producing the Fahrenheit scale with 32° for freezing-point, 96° (more correctly 98·4°) for body-heat, and 212° for boiling-point at ordinary atmospheric pressure. There are 180° between freezing and boiling points.

Réaumur’s scale has 80° between these points.

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]

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.

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).

Proof-spirit meant originally a spirit sufficiently strong to take light, and which, if poured on gunpowder and lighted, would cause the powder to explode. This was the ‘Holland-proof,’ By a statute of 1816 it was defined as of specific gravity 12/13 that of water at 51° F., which is = 0·923; but at the present standard temperature of 60° = 15·5° C. it is 0·920.

With Sykes’s hydrometer, used in England:

Under Proof (U.P.)—each degree means 1 per cent. of water, the rest being proof spirit.

Over Proof (O.P.)—each degree means 1 per cent. of water required to be added to bring the spirit down to proof.

The scale of Tralles’s alcoholometer only differs from Gay-Lussac’s by water being taken at 39·2° and pure alcohol as of sp. gr. 7939 at 60°.

The approximate relation of the five usual scales for proof and French rectified spirit are as follows:

Units of Power, of Electricity, of Caloric, &c., are abstract compound units based on units of time, weight, length, &c., combined for industrial convenience, e.g. a certain weight moved a certain distance in a certain time.

The H.P., engine horse-power, is 550 lb. raised one foot in one second. In France this becomes 75·9 kilos, raised one metre in one second, = about 3/4 of the kilo-watt unit of electric power.