90 PROCEEDINGS OF THE AMERICAN ACADEMY 



produces this large error at 0°, it follows that an error of only 0°.02 

 at 40° will produce an error of xsW at 0°. At other points the 

 errors will be less. 



Hence extreme care must be taken in the comparison and the most 

 accurate apparatus must be constructed for the purpose. 



2. Description of Apparatus. 

 77ie Air Thermometer. 



In designing the apparatus, I have have had in view the production 

 of a uniform temperature combined with ease of reading the ther- 

 mometers, which must be totally immersed in the water. The uni- 

 formity, however, needed only to apply to the air thermometer and to 

 the bulbs of the mercurial thermometer, as a slight variation in the 

 temperature of the stems is of no consequence. A uniform tempera- 

 ture for the air thermometer is important, because it must take time 

 for a mass of air to heat up to a given temperature within 0°.01 or 

 less. 



Fig. 1 gives a section of the apparatus. This consists of a large 

 copper vessel, nickel-plated on the outside, with double walls an inch 

 apart, and made in two parts, so that it could be put together water- 

 tight along the line a b. As seen from the dimensions, it required 

 about 28 kilogrammes of water to fill it. Inside of this was the vessel 

 md efghkln, which could be separated along the line die. In the 

 upper part of this vessel, a piston, q, worked, and could draw the 

 water from the vessel. The top was closed by a loose piece of metal, 

 o p, which fell down and acted as a valve. The bottom of this 

 inner vessel had a false bottom, c I, above which was a row of large 

 holes ; above these was a perforated diaphragm, s. The bulb of the 

 air thermometer was at t, with the bulbs of the mercurial thermometers 

 almost touching it. The air thermometer bulb was very much elon- 

 gated, being about 18 cm - long and 3 to o em - in diameter. Although 

 the bulbs of the thermometers were in the inner vessel, the stems 

 were in the outer one, and the reading was accomplished through the 

 thick glass window u v. 



The change of the temperature was effected by means of a Bunsen 

 burner under the vessel w. 



The working of the apparatus was as follows. The temperature 

 having been raised to the required point, the piston q was worked to 

 stir up the water ; this it did by drawing the water through the holes 

 at c I and the perforated diaphragm s, and thence up through the 



