Barrett — A Form of Open-Scale Isothermal Air Barometer. 446 



Instead of a short liquid index, another arrangement, whicli was found 

 convenient in practice, was to pour a little mercury — or other liquid having a 

 low vapour-pressure, such as creosote — into the Dewar flask, and allow the 

 lower end of the quill index-tube to dip into the liquid. The displacement 

 of the air in the flask, caused by pushing in the thick rubber cork, was found 

 sufficient to raise the level of the liquid to a convenient lieight in the index- 

 tube. 



My former lecture assistant at tlie Royal College of Science for Irelami, 

 Mr. George Hughes, made up one of these isothermal air barometers for 

 me more than a year ago, and has had it under observation ever since. He 

 has found its variations agree extremely well with the ordinary mercurial 

 barometer. The cost of such an instrument is trifling; and as a sensitive 

 weather-glass it may be found useful, although it cannot pretend to be an 

 instrument of precision, or to take the place of tlie ordinary barometer, 

 until temperature changes are wholly excluded. 



P.S. — Since the foregoing paper was read before the Society, my atten- 

 tion has been called to the fact that the publication of my new weather- 

 glass has been forestalled by a paper recently published in the Oomptes 

 Rendus. In this paper, read before the Paris Academy of Sciences on 

 December 6, 1909, M. Gr. Carpentier describes an isothermal air barometer, 

 made by the Marquis de Montrichard, which is practically the same as the 

 one I have here described and have had in use for over twelve months. A 

 model is depicted, resembling an aneroid barometer, in which the graduated 

 index- tube is coiled into a spiral form and separated by an opaque glass 

 division from the small Dewar air reservoir below. It is stated that the 

 inventor has lately made an ingenious addition to the apparatus whereby an 

 absolutely uniform temperature may be maintained in the air reservoir. 

 This consists in placing some ice inside the Dewar flask, or, more conveniently, 

 the ice may be dropped from outside into a little well or receptacle made in 

 the side of the air reservoir. No doubt, if the neck of the aperture to the 

 well be made small and plugged with cotton-wool, the ice will not need renewal 

 for some days; and if this be so, the apparatus may, for certain purposes, 

 become a useful laboratory instrument. 



