Definition of a MERCURIAL LEVEL, 15 



exact level. It may alfo be fixed on a tripod as the fpirit-level j 

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

 fingle (take, 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 pafTes through the hole G, and 

 through the eye or hole of the walking (lick ; 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: 

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

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

 fervation. 2dly, With this, the level of twenty different places 

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

 for one obfervation. sd/y, 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 exactnefs, 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. $thly t 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 



