650 PHILOSOPHICAL TRANSACTIONS. [aNNO 1723. 



transversely to the outer part of the upright board, where it is made cylindrical 

 is bent into the same figure, by means of four screw-pins, two of which passing 

 through each end of this, and of another piece of the same length, t, but 

 something narrower, placed over against it on the inside of the board, by 

 their nuts draw them together, so as to grasp the end of the upright board 

 between them; the upper edge of the rest being first shot with a plane very 

 straight and smooth. To render the motion of the arm along the rest smooth 

 and easy, it has two rollers lodged in a box fixed near the end, on its underside 

 V, to roll on the edge of the rest, when the end of the arm is moved along i. 

 One of the rollers is placed near each edge of the arm, and their axes lie in 

 lines passing through the axis of the turning pillar. The rest is kept up to 

 them, with a proper degree of force, by two screws, ww, which run in two 

 plugs, XX, fastened on the sides of the upright board, and bear against the 

 under sides of two pieces fixed on the inside of the rest. 



The motion of the tube is governed by two brass pegs, y and z. The first 

 of these, y, is placed about 10 or 11 inches from the end of the arm, and has 

 a line w^ound round it, which passing under a small pulley, f, fixed in a vertical 

 position near the end of the arm, is fastened to a staple on the under side of the 

 tube g. This line, by the turning of the peg, brings the fore end of the tube 

 to its due elevation, being acted against by the excess of weight in the hinder 

 end of the tube, when the metal is in it, which is equivalent to about 2 pound 

 at g, where the line is fastened. In great elevations of the object above the 

 horizon, the line is not carried so far as the point g; but is fastened a little 

 above the pulley, to a light square stick, h, having at one end a hook, by which 

 it takes hold of the staple g. This is done, that the springiness of the line 

 may not continue a vibrating motion in the tube, when any thing happens to 

 shake the instrument, and make the object appear to tremble. The lower part 

 of the stick rests against the end of the arm, and by its slight friction contri- 

 butes to the same effect. 



The other peg, z, is so placed, that it may be conveniently reached by one 

 hand of the observer, while the other is employed about the peg y; it regulates 

 the horizontal motion of the tube, by means of a line, which being wound 

 about the peg at one end, passes by another small pulley placed close by the 

 side of the aforementioned one in a horizontal position, not to be seen in the 

 figure, and is hung on a pin driven into the little head k. It is acted against 

 by two springs, m and n, fig. 13, placed in the box, in, one on each side of 

 the turning pillar ; that on the right hand, m, draws the right side of the pillar 

 forward, by a very strong line, which being fastened to the head of the spring, 

 passes round the back part of the pillar to a pin, at p, by which it is strained 



