I 



of an Air Barometer. 217 



vernier, and of reading off the observation, being so nearly 

 the same as in a portable barometer, need not here be parti- 

 cularly described ; and it is scarcely necessary to remark, 

 that since the surface of the included air in contact with the 

 mercury is so very small, the temperature of the mercury can- 

 not sensibly affect this instrument. 



If the air-ball be quite exposed to the air, and be at the same 

 time kept in the shade, it may be presumed that the included 

 air will be at least as near the temperature of the surrounding 

 air as the detached thermometer is ; and if so, an attached 

 thermometer may be dispensed with. Indeed, after all the 

 precautions that have been used, it may be questioned whe- 

 ther the thermometer attached to a mountain-barometer may 

 not sometimes differ considerably from tlie temperature of 

 the mercury in the barometer, especially when the two ther- 

 mometers themselves disagree. 



A difference in the temperatures of the included air at the 

 two stations, will affect the elevation so much more than the 

 same difference would in the temperature of the common baro- 

 meter, as (he effect of heat on air is greater dian on mercury. 

 Yet as the temperature of the air seems to admit of being 

 ascertauied with greater precision than that of tho mercury, 

 it may be presumed that this instrument will not on account of 

 heat be less to be depended on than the mercurial barometer. 

 If a lighter fluid could be employed in place of mercury, the 

 sensibility of the instrument might be greatly increased ; but 

 the evaporation, viscidity, capillary attraction, or some such 

 defect, almost precludes the use of any thing else. The range 

 or scale of this barometer might be made of almost any mag- 

 nitude, though it is doubtful if its sensibility can be inci'eased 

 Cjuite in the same proportion. Still, when of large dimensions, 

 its sensibility may much exceed that of the common barometer; 

 but a very large instrument would hardly deserve the name of 

 portable. It jnay however be at least as sensible as the mer- 

 curial barometer when only of about half its length. 



If the tube connected with the bulb, in place of being cylin- 

 drical, were to widen downward, so that the numbers on an 

 attached scale of equal parts might be the logarithms of those 

 already mentioned, the elevation could be obtained withgi'eater 

 facility: but the formation of such a tube with accuracy would 

 be a matter of some difliculty ; and unless the divisions are 



of compulinp; tlie elevation given by Dr. Robison, in which no tables are 

 re(|iiireil. Tlie only difference is, that here a correction is to be applied for 

 the temperature of the included air, instead of that of the mercury. 

 Vol. 6'J. No. 305. Sept. 1823. E e " equal. 



