Cuff. — The Cam-lever Balance. 



195 



for 20 degrees mark off 171 mm., and so on up to 90 degrees, 111-1 mm. 

 At the point so marked in each radius erect a perpendicular iucHhing 

 towards the side of lower magnitudes. These perpendiculars will inter- 

 sect at several points. Draw a curve tangential to these perpendiculars 

 on the inner side. This is the true curve required for the cam. The dis- 

 tance to be marked off on the radius F to 0^ is very approximately the 

 same as that given in the table for 1 degree — namely, 174-5 mm. — and this 

 point marks the lower termination of the curve. 



An examination of the true cam-curve reveals that a very close approxi- 

 mation thereto can be made in a much more simple way (see fig. 3). Call 

 the distance F to P, on the vertical side of the quadrant, f ; and the dis- 

 tance F to Q, on the horizontal side, q : then the ratio oi f to q must 

 always be as sin 90°/ 90° to sin =! 0°/ -i 0°. Taking 1 minute of arc, or 

 ,5^5 degree, as being =| 0°, then ji is to q as 0-Oi is to 0-0174505098. These 

 figures are almost perfectly in the ratio of 7 to 11. li f equal 7 units of 

 length and q equal 11 of the same units of length, then q will be 0-05BB per 

 cent, too long, or less than one part in 1776 too long. 



Hence the simple and practical way to mark off the cam-curve, approxi- 

 mately, is as follows : (1) Draw two lines, FP and FQ, at right angles ; 

 (2) make the vertical line FP equal to 7 units of length, of any suitable 

 size ; (3) make the horizontal line FQ equal to 11 of the same units ; 

 (4) mark a point H on FQ so that the distance from Q to H equals FP ; 



7*— Trans. 



