Mr. Nixon's Theory of the Spirit-Level. 257 



If we place in a vertical position the sides of a (glass) vessel 

 W, formed of two equal 

 parallel circular planes, f 



held together by a rim 

 R, perpendicular to the 

 planes, the surface of 

 contact of the incom- 

 pressible fluid (or liquid), 

 with the superincumbent 

 elastic fluid, together fill- 

 ing the vessel, will be sen- 

 sibly a horizontal plane. 

 A vertical plane passing 

 in the direction of the 

 centres of the circles, 

 (through their vei'tices 

 and that of the rim,) will 

 divide at right angles a 



straight line LZ, drawn on this horizontal surface parallel to 

 the circles, into two equal parts. The vertex or zenith v of 

 the circumference of either circle or the rim will therefore be 

 the point of bisection of such part of the arc of either as is 

 situated above this horizontal surface. 



Having marked this zenith-point, if the vessel be made to 

 describe in a vertical plane, any part of a revolution about 

 the horizontal line or axis C, passing through the centres of 

 the circles, the mark moving along with the vessel, will pass 

 over an equal arc of revolution. The zenith-distance of a 

 straight line drawn from the mark, which we will now call if, 

 to C, will therefore be equal to that arc or to the angle formed 

 by the intersection at C of this straight line, and a vertical 

 line passing through the new zenith-point of the rim, &c. found 

 by bisecting, as before, the arch of the rim, &c. now extended 

 over the horizontal surface L I. 



When the interior of the rim is perfectly circular, the arc 

 passed over (or zenith-distance of xl) may be measured at 

 once on its graduated parallel exterior. But should the figure 

 of the rim be that of any other curve, the length of L / will 

 vary in different parts of the curve; — the zenith-points will 

 seldom be vertical to the point of bisection of L /, or be si- 

 tuated at the middle point of the arch extended over it. The 

 points V and t/ must now be found exclusively by drawing 

 straight lines through the centre of revolution (or axis) of the 

 vessel perpendicular to LZ; their angular opening or zenith- 

 distance of t/ being measured on a graduated circle described 



licv) Series. Vol. I. No. 4. April 1827. 2 L on 



