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The Method of adjujiing the horizontal Pofttion of the Axis. 



Every one muft allow, that the adjuftment by the fpirit 

 level is of all modes the moft convenient, but it is not without 

 its defedts ; there are few levels true to lefs than two feconds in 

 winter, when all levels arc moft fluggifh; add to this, that the 

 pivots of the axis muft, for this adjuftment, be accurately of 

 equal diameters, and perfed cylinders ; we muft alfo take into 

 account the danger of any partial heat reaching the glafs tube, 

 which will create an enormous error. 



To remedy thefe defeds, a plumb-line has been fubftituted, 

 but generally not well contrived. The ufual adjuftment by the 

 plumb-line has been, to attach two fmall pins to the tube of the 

 telefcope, one near the objed glafs, the other near the eye end 

 of the tube, and to make a plumb-line, fufpended from one of 

 thefe, pafs over and bifed two fine points, one near each end of 

 the tube; then, inverting the telefcope, and fufpending the 

 plumb-line from the other pin, to fee whether it bifed the fame 

 points again ; and if not, to corred half the error in the ufual 

 manner : but this appears to be dangerous to the line of colH- 

 mation. 



My firft attempt at adjuftment by the plumb-line in the 

 year 1774, was to fufpend a brafs rod by hooks on the pivots, 

 like the horizontal bar of a level, and to the middle of this I 

 attached a vertical bar with two fine points, and a plumb-line ; 

 but this was fubjed to inaccuracy, unlcfs the pivots were pre- 



D 2 cifely 



