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It is univerfally allowed that mercury readily imbibes air 

 when in contaft with it, particularly when they are agitated 

 together : but as from the nature of the inftrument the con- 

 tad cannot be intirely avoided, our attention fhould be dired- 

 ed to lefTen the agitation as far as pofTible ; and to diminifli 

 the quantity of air in contad when fuch agitation cannot be 

 intirely removed. 



Most portable barometers which I have feen have the refer- 

 voir made of foft leather : From the pliable texture of this 

 material, and the great elafticity of mercury, a confiderable 

 degree of agitation takes place in carriage, even when the ad- 

 jufting fcrew at bottom is forced up as far as poiTible. — Now 

 even allowing the leather to be impervious to air, flill the 

 fmall quantity of air which remains in the cafes covering the 

 floating gages muft, when the inftrument is inverted, pafs 

 through the body of mercury in the refervoir, and be de- 

 tained by the leather in contad and perpetual agitation with 

 the mercury. 



Upon confidering this matter, it occurred to me that it 



might perhaps be of ufe to make one that fhould have the 



refervoir of wood : The following conftrudion has fucceeded 

 beyond my expedation : 



Plate ift, is a drawing of the inftrument, or rather of what 

 is different in this from other portable barometers. 



The upper part, which is here omitted, is contrived, as is 

 ufual in all good inftruments of this kind, to fhew the column 



of 



