Mr. D. Winstanley on Air-Thermometers. 381 



barometric variations of the external air. There is, of course, 

 no reason why the tube of such an instrument should be so 

 little or so much as thirty odd inches long. Neither is it ne- 

 cessary that mercury should be the liquid used. Sulphuric 

 acid, for instance, will answer just as well. It is not even 

 needful that the liquid should be non-volatile at the tempera- 

 tures to which the apparatus is exposed. The objection which 

 exists to the employment of a volatile liquid in the ordinary 

 barometric column does not here apply ; for that depression 

 which the vapour tends to produce in one limb it tends to pro- 

 duce in the other one as well, and so leaves the equilibrium 

 of the liquid undisturbed, and dependent only on the expan- 

 sion and contraction of the air. It is merely needful, then, in 

 this form of air-thermometer, that the gas enclosed shall be 

 submitted to the definite pressure of an isolated barometric 

 column ; and we may employ what liquid and what length of 

 tube we please. We may even construct a veritable air-ther- 

 mometer without a barometric column by resorting to some 

 other means of obtaining the definite pressure we require ; and 

 to this end the author has employed a vacuous corrugated 

 elastic box similar to those contained in aneroid barometers ; 

 but he has used it with the spring inside. This thermometer 

 is shown in fig. 1. At the bottom we have a rigid box with 

 the elastic one enclosed. The rigid one is sealed ; and the elastic 

 one is surrounded by a liquid which rises some distance up a 

 tube, in which the indications will be made. The tube is sur- 

 mounted by a bulb of air. This air, with a rise of tempera- 

 ture, expands and forces down the substance of the liquid 

 column into the space made for it by the compression of the 

 elastic box; and with a fall of temperature the reverse of 

 course takes place. 



The air-thermometer which I first described (that with the 

 isolated barometric column) cannot, in the shape in which I 

 have described it, be regarded as having a convenient form, 

 inasmuch as through the greater portion of its length the 

 liquid will never move. Obviously the index of the barome- 

 tric column will not reach the level of the liquid in the cistern 

 until the rational zero is attained ; and the amount of space 

 which marks the range between the freezing- and the boiling- 

 points of water will have nearly three times as large a space 

 below, devoted for the greater part to readings which will very 

 likely never be observed. 



I find, however, a very convenient instrument may be pro- 

 duced if, in its construction, two liquids are employed, as 

 shown in fig. 2. There we have a mercurial barometric 

 column. Its cistern is connected with the bulb of air by a 

 Phil Mag. S. 5. Vol. 10. No. 63. Nov. 1880. 2E 



