16 Defcription of a MERCURIAL LEVEL. 



Th e following is a proof of the exadmefs of this method. John 

 Miller, the mathematical inftrument- maker, has a line drawn 

 upon the oppofite fide of the Parliament- fquare, fronting his fhop, 

 by which he has been in ufe to adjuft his fpirit-levels. We placed 

 the mercurial level upon the fpot known to be upon an exact 

 level with the line. Both he and I looked through the fights °, 

 but could not perceive the line. We fufpected there was fome 

 fault in the rights ', but, on making them vibrate, we found that 

 the hair had covered the line ; fo foon as they fettled, the line 

 was again covered by the hair. 



When there is a ftrong wind, the fights vibrate too much. 

 In order to remedy this, there is a cafe of tin'd plate or pafte- 

 board made to inclofe the inftrument when not in ufe. Vid. fig. 5. 

 When ufed, the cafe covers only about one half of it, leaving 

 room for the fights to float within the cafe. There are two oval 

 holes at each end of the cafe through which the obfervations 

 are made. 



IV. 



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