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way of making a fcale we avoid the trouble of meafuring 

 exadly the diameters of the box and of the tube, and of its orifice 

 Oft. bore, and of finding out from thence what is to be the length 

 of our correded fcale. Inftead of this we have only the length 

 of one fpace or line to meafure, and this gives the length of our 

 fcale without any calculation. It is fo convenient to have a 

 corred fcale, fuch as I have mentioned, for a barometer, and 

 the method of making one is fo fimple and obvious, that 

 we may wonder it has not long fince been known and prac- 

 tifed. 



We fee that, according to this fcheme, every portable 

 barometer muft have a fcale made purpofely for itfelf, and a 

 vernier adapted to that fcale ; fo that to get fuch a fcale made 

 we muft befpeak it, and tell the proportion we would have 

 its aliquot parts bear to thofe of the common inch-fcale. If it 

 be thought that this is any inconvenience, and that it would 

 be defirable that all portable barometers fhould ufe one common 

 fcale, which might be had ready made with a vernier adapted ; 

 this is a thing that may be eafily effeded, if it was generally 

 agreed what the length of that common fcale fhould be. I would 

 therefore propofe, for inftance, that the length of the fcale fhould 

 be -'- lefs than the fcale of three inches now in ufe, which would 



*^ 5 o 



be no great diminution. And in this cafe an artificer would 

 have a very eafy rule by which he might fo conflrud his baro- 

 meters, that the fcale now propofed fhould anfwer for them 

 all. The rule is this, meafure the external diameter of the 

 tube you intend to ufe, and the diameter of its orifice or bore ; 



O 2 make 



