Thermometer Scales - 1910 Travel Guide
Much annoyance is caused by the great difference of thermometer scales in use in the different civilized countries. The scale of Reaumur prevails in Germany. As is well known, he divides the space between the freezing and boiling points into 80 deg. France uses that of Celsius, who graduated his scale on the decimal system.
The most peculiar scale of all, however. Is that of Fahrenheit, 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 therefore did not take the congealing point of water, but composed a mixture of equal parts of snow and sal ammoniac, about —14 deg. R.
The conversion of any one of these scales to another is very simple, and easily made. To change a temperature as given by Fahrenheit's scale into the same as given by the centigrade scale subtract 32 deg. 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 32 deg. from Fahrenheit's degrees, and multiply the remainder by 4-9. The product will be the temperature in Reaumur's degrees.