Defcription of a MERCURIAL LEVEL. 15 



exadl level. It may alfo be fixed on a tripod as the fpirit-level ; 

 or it will anfwer equally well, if it is affixed to the top of a 

 fingle ftake, which is fharpened at the point fo as to be pufhed 

 into the ground. If it is to be ufed as a pocket-inftrument, it 

 may be made of feven inches length, being about double the 

 dimenfions of the annexed draught. A common walking cane 

 forms a very convenient fupport. It is affixed to the cane by 

 means of a brafs pin E, which pafles through the hole G, and 

 through the eye or hole of the walking ftick ; and a brafs nut 

 F, fcrewing to the male-fcrew of the brafs pin, keeps them firm 

 together. The two grooves B B, contain the two fights and 

 brafs pin, when not in ufe. Two corks, covered with thin lea- 

 ther, fitted into the holes A A, confine the mercury, when the 

 inftrument is to be tranfported ; or, in cafe the mercury is 

 found to efcape, it may be poured into a fmall cafe, made of 

 lignum vitae, like a tooth-pick cafe ; and this may be flopped 

 with a cork, and made to fit into one of the grooves. 



The advantages of this inftrument over the fpirit-level are: 

 I/?, It requires no adjuftment, confequently two obfervers, 

 though otherwife not equally accurate, muft make the fame ob- 

 fervation. ^dly. With this, the level of twenty different places 

 may be taken during the time required to adjuft the fpirit-level 

 for one obfervation. 3^/^, The nicety of the fpirit-level depends 

 upon the fmall curve of the glafs-tube, in the choice of which 

 no rule can be laid down ; neither is any thing gained, in point 

 of exadnefs, by lengthening the fpirit-tube above three or four 

 inches. But every inftrument of this kind is of one ftandard ; 

 and the further the two fights are removed from one another, 

 the more any error is diminifhed. iifthly. This inftrument can 

 be made perfectly juft, without taking any obfervation, or com- 

 paring it with another level. In order to do this, let the floats 

 on which the fights reft, be of the fame dimenfion and weight,, 

 and let the crofs-hair and eye-hole be of one height, and, with- 

 out farther adjuftment, they will point out the true level. 



The. 



