THE FIRST THERMOMETERS. 689 



Archimedes." Everybody was curious to observe the asceut or 

 descent of the colored spirit in the tube; for, Nollet wrote, "the 

 physician, guided by the thermometer, can labor with more cer- 

 tainty and success; the good citizen is better informed regard- 

 ing the variations that concern the health of men and the produc- 

 tions 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." Ac- 

 cording 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 com- 

 pare 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 subordi- 

 nated. 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 main- 

 tain 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 



TOL. LII. 51 



