The Fahrenheit thermometer, in general use throughout the British Empire and the United States, was introduced in 1714, and is an adaptation of the thermometer invented by Sir Isaac Newton, and described by him in the Philosophical Transactions for 1701. Newton's lowest point was that of freezing, his highest that of boiling water, his starting point, however, being the heat of the human body, which he called the round number 12, the duodecimal system being then in use; that is, he divided the space between the freezing point and the temperature of the body into twelve parts. When Fahrenheit took the thermometer in hand he divided each of Newton's degrees into two parts, so getting a more minute record, while he got lower temperatures than freezing by using a mixture of ice and salt for his zero. Newton used linseed oil as the fluid, but mercury came into general use as more convenient. The boiling point of a Fahrenheit thermometer is 212º, the freezing point 32º.

The centigrade thermometer, in general use in France, has 0º for its freezing point and zero, and 100º for boiling point. The temperatures below freezing point are indicated by a minus sign - prefixed to the degree number. Thus -12 C means 12 centigrade degrees of frost, while -12 F means 12 degrees below 0º, or 44 Fahrenheit degrees of frost.

The German thermometer, the Réaumur, gives 0º as the freezing point, and 80º as the boiling point.

Readings of any one of these three scales can be converted into those of any other by the following rules:--