Page:Popular Science Monthly Volume 52.djvu/709

Rh Archimedes." Everybody was curious to observe the ascent or descent of the colored spirit in the tube; for, Nollet wrote, "the physician, guided by the thermometer, can labor with more certainty and success; the good citizen is better informed regarding the variations that concern the health of men and the productions of the earth; and the individual who is trying to procure the conveniences of life is informed by it as to what he must do in order to live all the year in a nearly uniform temperature." According to Amontons, Colbert had a project for constructing a large number of thermometers and sending them to different parts of the earth for making observations on seasons and climates, but was obliged to give it up on account of the imperfect character of the spirit thermometer of the time. Different instruments would not agree.

The marking of the degrees on the thermometer stems was not controlled by any fixed rule, and they therefore did not express the same heat or the same cold by the same number of degrees. To remedy this defect, some physicists advised that the lowest point reached in the extreme cold of winter and the highest in summer be marked, and the space between be divided into a hundred equal parts. Such a thermometer would indeed permit its owner to compare the cold and heat of different years; but in communicating his observations to another he would give him data that would have no meaning unless he also sent him the instrument he had used, or one having identical graduations.

The problem was first solved in 1702 by Amontons; and his method, although it has been given up and resumed at intervals, has now become the normal one to which all others are subordinated. It is based upon two observations, both of which are of primary importance. We take two masses of air in two bulbs. Each of these masses is separated from the outer air by a curved tube filled with mercury, forming a manometer. Suppose that at a given temperature one of these masses supports a pressure of one, and the other of two atmospheres. Warm the two masses of air equally, and pour into both manometers enough mercury to maintain invariable the volume occupied by each of them. While the pressure supported by the first mass will increase to a certain amount, that sustained by the other mass will increase doubly. The pressure on the second will always be double that on the first. Thus, when we warm the two masses equally, while keeping invariable the volume of the recipients containing them, a constant relation will be maintained between the pressures supported by them. This is Amontons's first observation.

In the second observation, which can be made with an arbitrarily