COMPARISON OP PLATINUM AND i;.\^ I HKKM<>MKTER8. 71 



tube and placed at a distance of 38 centims. from the manometer. A second tube 

 serves to support three other small telescopes, for the observation of the scale-vernier, 

 and two auxiliary mercury thermometers, which are placed close to the mercurial 

 columns to indicate their temperature. 



During the measurements the observer is at a distance of about 50 centims. from 

 the apparatus ; his influence on the temperature of the mercurial columns is thus 

 considerable, as is also that of the various heating baths, and by reason of the great 

 expansion of mercury, this heating probably constitutes one of the principal sources 

 of error in the experiments. To diminish as far as possible the radiation from the 

 comparison-bath, a double walled metallic screen, in which a current of water 

 circulated, was interposed between it and the manometer. 



(e) Divided Scale. 



The divided scale used was constructed by the Socie"te GdneVoise, and has served 

 for previous work with the gas thermometer. Its length is 1'5 metres, and its cross- 

 section is in the form of an H. This H-fonn is not well adapted for use with a 

 vernier. It would l>e letter to adopt a form allowing the surfaces carrying the 

 divisions of the scale and vernier to be in the same plane. In this scale the divisions 

 are on a plate of silver let into the median transverse face, very near the plane of the 

 iiriit nil HI ires. 



Fig. 14 shows in horizontal projection the disposition of the pieces which supjxn-t 

 the scale and attach it to the barometer, and fig. 13 the pieces which hold the vernier 

 on the supjKtrt of the manometer tube, and which ensure contact between the scale 

 and the vernier. The method of suspension of the scale permits it to turn about two 

 axes perpendicular to its length. In the two directions of free movement two springs 

 gently press the scale against the vernier. 



Two thermometers placed at equal distances from the points in the barometer and 

 manometer tubes, the one on a fixed support, the other on an attachment to the 

 Kn-ometer, serve to indicate the temperatures of the mercurial columns and of the scale. 

 Each of these thermometers is placed in a test-tube filled with mercury, of the same 

 diameter as tin- neighlxmring portion of the manometer tubes. This symmetrical 

 iirrangement of the thermometers with regard to the ends of the mercury columns 

 whose temperature is to be measured considerably simplifies the calculation of the 

 mean temperature of the manometer. 



XXVII. ZERO APPARATUS. 



A -I.-.S- IH-H-J.-II -,u|i|),,it..,i ,,n ,ui in .I, in),,,,! .-,11.1 MnMbded I'.v -.<!.'! hjen of 

 (Mt serves as the receiver. The ice, finely divided and saturated with pure water, is 

 pressed around the reservoir of the thermometer, the emergent stem being held by a 



