380 Mr. D. Winstanley on Air-Thermometers. 



Hence the temperature in these narrow tubes is already 

 very low; in one ten times as wide it might sink to 100° C. 



This is a further confirmation of the previously stated law 

 that a gas may be rendered luminous by electric discharges with- 

 out any corresponding elevation of temperature. 



[To be continued.] 



XLIII. Air-Thermometers. By D. Winstanley*. 

 [Plate VII.] 



A THERMOMETER which makes its indications in con- 

 sequence of the dilatation and contraction of a gas offers 

 several advantages over one which depends therefore on the 

 volumetric variations of a liquid. Gases under constant pres- 

 sure expand considerably more than liquids do for the same 

 elevation in their temperature. Hence an air- or gas-thermo- 

 meter, having the same size of bulb and tube as one in which a 

 liquid only is employed, will have a more legible and open 

 scale. Again, a given volume of a gas at the ordinary barome- 

 tric tension of the air upon the level of the sea, when compared 

 with an equal volume of a liquid body, requires so utterly 

 insignificant an amount of heat to elevate its temperature 

 through a given range, that a gas-thermometer is enormously 

 more sensitive than one which depends upon a liquid for its 

 expansional effects. And, finally, the very equal manner in 

 which gases are dilated under the influence of equal increments 

 of heat is a very strong point in favour of an air-thermometer. 

 As constructed by Galileo, the air-thermometer unfortu- 

 nately gave readings which were influenced by the barometrie 

 variations of the outer air, a circumstance which has limited 

 considerably its use and application. Happily it is not diffi- 

 cult to construct an instrument which shall be free from 

 this defect. If we take an ordinary mercurial barometer 

 made after a certain well-known pattern, i. e. with a bulb- 

 shaped cistern surmounted by a neck into which we may 

 insert a cork, and if as a matter of fact we do insert a 

 cork, obviously that barometer ceases to show the tension 

 of the outer air, and is a barometer only to the air enclosed 

 within its bulbous cistern. But as the tension of this air will 

 vary with its temperature, the height of the mercurial column 

 will vary therewith as well ; and that which was a barometer 

 will, by the mere insertion of a cork, have become an air- 

 thermometer, the readings of which are uninfluenced by the 



* Communicated by the Physical Society, having been read at the Meet- 

 ing on June 26. 



