[ '^S ] 



proportional, and make it one fide of a right-angled triangle, 

 and let the other fide be equal to the diameter of the tube. 

 The hypotenufe will be the diameter that the box of the 

 barometer ought to have, in order that the propofed fcale may 

 be the proper fcale for it. 



Portable barometers have the advantage of being filled with 

 lefs trouble than the common ones ; for when the tube is filled, 

 we have nothing more to do than to pour into the box as much 

 mercury as we are fure will cover the orifice of the tube, 

 in whatever pofitions the inftrument may be placed, and then 

 ferew the cover on the bottom of the box with a collar of 

 leather to prevent the mercury from getting at the threads of 

 the fcrew. The upper part of the box, which is folid, ought 

 not to be lefs than | of an inch in length, that it may take 

 a fufficient hold of the tube cemented into it. The end of the 

 tube fhould go into the cavity of the box fo far as the middle 

 of its length, and we ought to pour into the box as much 

 mercury as will leave only i of an inch in length to be occupied 

 by the air when the barometer is ered ; this fpace will be 

 fufficient to allow the mercury in the tube to fall through ten 

 or twelve inches, which will be full enough for meafuring 

 the heights of any places to which we ufually have accefs, and 

 we may be then fure we have put in as much mercury as will 

 cover the orifice of the tube in any pofition of the inftrument. 

 One reafon, I believe, why it was thought neceffary that air 

 Ihould be excluded from the box of a barometer, while it was 

 carried from one place to another, was, that the mercury would 



be 



