CHAPTER XV
MEASURES OF HEAT AND OF DENSITY
AND COMPOUND INDUSTRIAL UNITS
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]]