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two or three thoufand feet above the level of the fea, of marking 

 regularly and truly the feveral inequalities of the grounds, and 

 of fhewing their adual differences of heighth within a few inches, 

 by an inftrument of no comparative price, that requires in no 

 cafe whatever the fmalleft adjuftment, hardly any previous fkill 

 in its application ; is liable to fcarce any accident or error, and 

 is not more cumbrous than a common walking ftafF. 



If it fhall appear that fuch an inftrument, within the limits 

 afligned, is capable of extreme accuracy, and anfwers equally 

 well all the purpofcs of the complicated, expenfive and operofe, 

 I fhould hope that this improvement would be favourably received 

 by the more Ikilful and learned, and afford an incitement toothers 

 to enter upon an eafy experiment that may lead to many valuable 

 difcovcries in the courfe of their refearches. 



I SHALL now proceed to the manner of making and ufing this 

 kind of portable barometer, and fliall add fome remarks on the 

 peculiar and confiderable advantages of its conftrudion. It may 

 be proper to premife that the principle on which this inftrument 

 adls is this ; that corkwood is a fubftance, the pores of which 

 afford a ready and free pafTage to the particles of air, while at 

 the fame time they are too fmall to fuffer thofe of quickfilver to 

 to efcape, except indeed forae particular and powerful means arc 

 ufed to force them through its interftices. 



The barometer confifts of a tube not much more than thirty 



inches long, an ivory cylinder about two inches in length, 



and upwards of one inch in diameter, open at one end, clofed 



Vol. V. N at 



