Between the graduation of the freezing and the boiling point of water the space is divided into 180 parts, which added to 32 make up the boiling point of water to 212 degrees, being the graduation of Fahrenheit, who was an instrument-maker of Hamburg. Why he divided the space between the freezing and boiling point of water nobody appears to know, unless he took a half circle of 180 degrees as the best division of space. If the thermometer contains air the mercury divides itself frequently into two or three slender threads, each separated from the other in the capillary bore, and thus the instrument is rendered useless until the threads again coalesce. If the thermometer has been well made, and is quite free from air, it may be tied to a string and swung violently round, when the centrifugal force drives the slender threads of mercury to their common source—viz., the bulb containing the quicksilver, and the whole is again united. The string must be attached, of course, to the top of the thermometer scale.
When travelling on the Continent it is sometimes desirable to be able to read the thermometers which are graduated in a different manner to that of Fahrenheit. In France the Centigrade scale is preferred, and in many parts of Germany Reaumur's graduation. The difference of the graduation is seen at a glance.
| In the | Centigrade | the freezing point is | 0, | the boiling point | 100°. |
| " | Reaumur | " | 0, | " | 80°. |
| " | Fahrenheit | " | 32°, | " | 212°. |
The number of degrees, therefore, between boiling and freezing is 100 in the Centigrade, 80 in Reaumur, and (212-32, that is) 180 in Fahrenheit.
If, then, the letters C, R, F, be taken to denote the number of degrees from the freezing point at which the mercury stands in the Centigrade, Reaumur, and Fahrenheit thermometers, we have the following proportions:—
(1.) 100: 80 :: C: R, whence C = 5/4 of R, or R = 4/5 of C.
(2.) 180:100 :: F: C, whence F = 9/5 of C, or C = 5/9 of F.
(3.) 180: 80 :: F: R, whence F = 9/4 of R, or R = 4/9 of F.
The following examples will show how to apply these formulæ:—
(1).—Suppose the Reaumur stands at 28°, at what height does the Centigrade stand? We have C = 5/4 of R (in this case), 5/4 of 28 = 35: that is, the Centigrade stands at 35°.
(2).—Suppose Fahrenheit to stand at 41°, what will Reaumur stand at? R = 4/9 of (41-32) (that is, the number above freezing in Fahr.) = 4/9 of 9 = 4. Reaumur stands at 4.