[ 374 ] 



to fuch numerical part or multiple of the fum of the furrep- 

 titious and unextraded air during one flroke, as is expi-efled 



in 



tvveen the height of the mere" in tlie latter and that in th.e mod perfe£l (landard- 

 baromr; becaufe the difFerence in their altitudes is not the hundredth part of an 

 inch ; fo tliat greater degrees of raref.iclion can be known only by the pear-gage ; 

 which fhouUl therefore be ufed 5 and it fliould be of a proper form and accurately 

 (Graduated. It is a glafs-vefiel in (hape fomewhat like a pear or rather an hydro- 

 meter, with a bulb, of a fize to contain between four and fix ounces of mere'', 

 terminating on either fide in a tube : that below the bulb (about an inch in length) 

 is open ; the upper part or ftem (about five or fix inches long) is fealed hermetically 

 at top ; the internal diameter of one half of the length of the ftem next the bulb, 

 fliould be about ith of an inch, and the cavity exadlly cylindrical, as likewife muft 

 be that of the upper half ; but this latter fhould be as narrow in the bore as a middle- 

 fized mercurial thermometer, viz. about ^^th of an inch in diameter : if the ftem 

 were of the fame diraenfions throughout, it could not, unlefs made inconveniently 

 long, meafure both fmall and great degrees of rarefaction : it is eafily made of the 

 above form, by blowing a bulb on a piece of glafs-tube, about ^th of an inch in 

 diameter; then drawing out the upper part of it (by the glafs-blower's lamp) into 

 a {lender tube, and fealing its end : or rather by adding to the tube a piece of a 

 thermometer tube. The ftem is faftened in a brafs pipe or cafe, having a wide 

 flit along its whole length, through which the mere' in the ftem is feen, and the 

 degrees are marked on the cafe. During the exhauftion, the gage is fufpended 

 by a flip-wire with its open end over a ciftern of mere' within the rec, fo tljat 

 the air in it is as much rarefied as that in the rec ; and when the pump his 

 been wrought zs much as is thought proper, the end of the gage is immerged ia 

 the mere'', which on the gradual readmifhon of the air, will be forced up into the 

 gage, and fill the bulb ; all the air which had been left in the gage, rifing to the 

 top, and being reduced nearly to the denfity of the external air ; which it would be 

 exaftly ; if the gage were placed not vertical, but horizontal. Then, as the whole 

 cavity of the gage, is to the part at the top filled only with air ; fo is the primi- 

 tive air v.-hich wr.s in the gage, and alfo in the rec, to the laft refiduum in each 

 refpedlively, or fo is the rarefaction produced -, hence, the ratio of the whole con- 

 tent 



