| 28.81 | 28.51 | 9 } | | 28.70 | | 10 } | | | |The boiling point to be | | | |marked so much higher or| | | |lower than the stand of | | | |the mercury during the | | | |experiment. | +-----------------+-----------------+------------------------+ The zero, 0°, of the thermometer of Fahrenheit, is taken by means of a mixture of snow and common salt, and its maximum point is, like that of the preceding thermometer, taken by means of boiling water; but this interval is divided into 212 degrees; so that the scale marks 32° where the centigrade and Réaumur’s scales mark 0°. The thermometer of Delisle has but one fixed point, which is the heat of boiling water; this is the zero of the instrument. The inferior degrees are 0,0001 (one ten-thousandth part) of the capacity of the bulb and stalk of the thermometer. It marks 150° at 0° of the centigrade, or 32° of Fahrenheit’s thermometer. The dial, the maximum and the minimum thermometers, are graduated according to the same principles as the common thermometers. You can, with a mercurial thermometer, make the centigrade scale rise to 300 or 400 degrees above zero; but with an alcohol thermometer, you must never go beyond the heat of boiling water. On the contrary, the inferior degrees of the alcohol thermometer can be carried to the very lowest point, while those of the mercurial thermometer should be stopped at thirty or thirty-five degrees below the zero of the centigrade scale, as the mercury then approaches very near the point of its congelation. In all cases, the degrees of thermometer scales are indicated by the sign - when they are below zero, and by the sign + when they are above it; the -is always marked, but the + generally omitted. See pl. 4, fig. 6. We may observe here that it is proper from time to time to plunge the standard thermometer into melting ice, for the purpose of verifying its exactness. It has been found that thermometers constructed with a vacuum above the column of mercury gradually become inaccurate, the 0° ascending, until it corresponds with + 1° or + 2°. This singular effect is attributable to the constant pressure of the atmosphere, which, being supported merely by the resistance of the very thin sides of the thermometer, finally presses them together, and diminishes the capacity of the reservoir. It is partly for the sake of avoiding this inconvenience that we consider it good not to make an entire vacuum above the mercury, but to leave a portion of air in the tube, and at the same time to form a little reservoir at the summit of the instrument.