[ t24 ] 



make a right-angled triangle, one fide of which fhall be equal 

 to the diameter of the tube, and the other fide feven times 

 the diameter of the bore, the hypotenufe will be the proper 

 diameter for the box, fo that the fcale now propofed fhall be 

 a corredt fcale for that barometer. The reafon of this is plain : 

 For in the barometer, thus conftru£ted, the fquare of the diameter 

 of the box is equal to the fquare of the diameter of the tube, 

 and alfo to 49 times the fquare of the diameter of the bore ; 

 therefore (fince circles are as the fquares of their diameters) 

 the area of the box is equal to the area occupied by the lower 

 end of the glafs tube, and to 49 times the area of the bore 

 of the tube. And therefore the annular area in the box, 

 occupied by the furface of the mercury, is 49 times the furface 

 of the mercury in the tube, confequently it is to the fum of 

 thefe two furfaces as 49 to 50, and therefore it follows, from 

 what has been faid, that the propofed fcale, whofe length is 

 to that of the common fcale as 49 to 50, will be the proper 

 corred fcale for this barometer. 



The foregoing rule, when expreffed in general terms, will 

 dired us how to conftrud a portable barometer, whofe contraded 

 or corred fcale fhall bear any given proportion we pleafe to 

 the common fcale of inches. Take two numbers, differing 

 by a unit, the lefTer of which fhall be to the greater in the 

 proportion we intend the contraded fcale fhall have to the 

 common one : then as a unit is to the lefTer of thefe numbers, 

 fo let the diameter of the bore of the tube be to another line ; 

 between this line and the diameter of the bore find a mean 



proportional, 



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