There are three standard thermometers in use, each a recognized standard in one or another part of the world. The scale of Reaumur (R.) prevails in Germany. As is well known, he divides the space between the freezing and boiling points into 8o°. France uses that of Celsius (C.), who graduated his scale on the decimal system. The most peculiar scale of all however, is that of Fahrenheit (F.), a renowned German physicist, who in 1714 or 1715 composed his scale, having ascertained that water can be cooled under the freezing point without congealing. He did not take the congealing point of water, which is uncertain, but composed a mixture of equal parts of snow and Sal Ammoniac, about—140 R. This scale is preferable to both those of Reaumur and Celsius, or, as it is called, Centigrade, because: 1. The regular temperatures of the moderate zone move within its two zeros, and can therefore be written without + or—. 2. The scale is divided so finely that it is not necessary to use fractious, when careful observations are to be made. These advantages, although drawn into question by some, have been considered so weighty, that both Great Britain and America have retained the scales, while the nations of the Continent use the other two. The conversion of any one of these scales into another is very simple. 1. To change a temperature given by Fahrenheit's scale into the same given by the Centigrade scale, subtract 32° from Fahrenheit's degrees and multiply the remainder by 5-9. The product will be the temperature in Centigrade degrees. To change from Fahrenheit's to Reaumur's scale, subtract 320 from Fahrenheit's degrees, and multiply the remainder by 4-9. The product will be the temperature in Reaumur's degrees. 3. To change a temperature given by the Centigrade scale into the same given by Fahrenheit, multiply the Centigrade degrees by 9-5, and add 320 to the product. The sum will be the temperature by Fahrenheit's scale. 4. To change from Reaumur's to Fahrenheit's scale, multiply the degrees on Reaumur's scale by 9-4, and add 320 to the product. The sum will be the temperature by Fahrenheit's scale.