The two thermometers most generally in use at present among the philosophers of Europe, are those or Reaumur and Fahrenheit. The French use Reaumur’s, the English Fahrenheit’s.

In their respective graduations, Reaumur marked his freezing point 0, Fahrenheit fixed his at 32 of his degrees above 0, and two of his degress are just equal to one of Reaumur’s. I know that in some instruments this equality is not exact; but in two which I have, the one Reaumur’s made by Cappy in Paris, the other Fahrenheit’s by Nairne, London; it is precisely so, they hanging together in the same room. And those workmen are famed for their exactness.

In reading, one frequently finds degrees of heat and cold mentioned, as measured by one or the other of those thermometers, and one is at a loss to reduce that least known to the other.

Suppose the degree mentioned is 25 of Reaumur, which is 25 degrees above 0, or his freezing point, and you would know to what degree of Fahrenheit that answers.

Double the 25, which will give you 50 of Fahrenheit’s, and to them add 32, his number at the freezing point, and you will have 82, the degree of Fahrenheit’s equal to 25 of Reaumur.

On the contrary, if you would reduce Fahrenheit to Reaumur, first subtract 32, the take half of the remainder; thus taking 32 from 82, there remains 50, and the half is 25.

This answers in all cases where the degree is above the freezing point.

If below, double the degrees of Reaumur, and subtract them from the 32 of Fahrenheit, which will give you the equivalent degree of his scale. Thus suppose it 5 below 0, or the freezing point of Reaumur; twice five is 10, which deducted from 32, Fahrenheit’s freezing point, give you 22 as the equivalent degree of his thermometer.

And halving the degrees of Fahrenheit that are less than 32, you have the degree of Reaumur. Thus 22 of Fahrenheit being 10 degrees less than 32, the half of 10 is 5, the equivalent degree of Reaumur.