[ "8 ] 



the outward air and that in the box. And even fuppofing the 

 pores of t'lc cork were at firft fufhciently permeable by air, 

 yet they may be in time obftruded by duft, the cork may imbibe 

 moifture which will contrad or flop its pores ; and, as there is 

 no hole to drop in a floating gage, you cannot at any time 

 meafare accurately the height of the mercury, or be fure it is 

 the liirfie that it would be were the barometer open. I would 

 therefore recommend that, inftead of a cork, the top of your 

 box fliould be of ivory, with a hole to drop in a floating gage, 

 which is the cafe in all other portable barometefs. This hole 

 you may occafionally fl;op with a peg or fcrew, and then the 

 inftrument will be fafely portable : or perhaps it might be 

 better to have a cover to fcrew over the top of the box, and 

 a hole in it to correfpond with the one in the box. AVhen 

 thefe two holes are together the box is open ; and it is fliut 

 when the holes are removed from each other by turning the 

 cover and fcrewing it tight to the top of the box, and if there be 

 a plate of foft leather between them, it will be fufiicient to 

 keep in the mercury when the infl;rument is agitated by carriage. 

 That I might let you know whether this fcheme would fucceed 

 I have had a barometer made in this form, and find it anfwers 

 all the puipofes of an open and of a portable one. The tube is 

 not inclofed, like your's, in a mahogany ftaff", but fitted in a frame 

 of the ufual form. There have been various other methods 

 propofed for making barometers portable, but all thofe I have 

 met with are of a confl;rudion more complex than is neceflary. 

 I have feen one made for the late Dodor Ufsher by Nairne and 



Blount, 



