III. Description of a Mercurial Level, invented by 

 Alexander Keith, Efqj F.R. S. & A. S. Edin.*. 



FIgure t. is a fection of the inftrument formed of maho- 

 gany or boxwood. A A are two oblong fquare cavities 

 connected together by a narrow clofe channel, running from 

 the bottom of the one to the other. B B are two grooves hol- 

 lowed out of the wood, in order to contain the fights, i$c. 

 They are fliut up by a lid, which turns upon a fcrew-nail at the 

 centre C, as may be feen more diftinctly from fig. 4. 



Fig. 2. DD are the two fights, the one with a fmall hole, 

 the other with a crofs-hair. Thefe fights are erected upon two 

 pieces of ivory or hard wood, which are fhaped nearly of the 

 dimenfions of the cavities A A, but fo much fmaller as to enter 

 without touching or rubbing on the fides. Mercury is poured 

 into the two holes A A till they are about half full ; the two 

 pieces of ivory which fupport the fights, are put into the cavi- 

 ties, and float on the furface of the mercury. 



Fig. 3. is a perfpective view of the inftrument when the 

 fights are floating upon the mercury ; and fig. 4. is another 

 view of it, when the fights are taken out and the lid is open. 



As the two cavities communicate with each other, the fur- 

 face of mercury in both are always upon the fame line of level; 

 and confequently, if the two fights are once accurately adjufted, 

 they will ever after point out the true level, without requiring 

 any after adjuftment. 



When this inftrument is to be ufed, it may be laid on any 

 horizontal furface, and the fights will immediately become an 



exact 



.* This Paper was read before the Philofophical Society of Edinburgh in December 

 1778 j and is now printed by order of the Committee for publication of the Tranfadtions 

 of the Royal Society of Edinburgh. 



