Form of Harmonic Analyser. 



373 



along this rule : it is provided with a tracer T and some scales 

 of sines S S. The whole sheet of card is guided parallel to 



Fie;. 4. 



the axis of x by a T-square DAR clamped to the drawing- 

 board : if the card is pulled forward by the corner A the 

 friction keeps it set against the square. P is a planimeter 

 with its pole fixed to the card and its pointer P resting on one 

 of the scales of sines, which must stand parallel to the axis of x. 



Mark off along the base of the curve to be analysed a 

 number of equal divisions, e. g. 6° each, and erect the ordi- 

 nates y 1} y 2y &c. at the centres of each of these elements. 

 Suppose the pointer P of the planimeter to rest initially at 

 the zero of the scale, and the tracer T attached to the set 

 square to stand over the origin of the curve. Pull the card 

 forward, carry T to the top of y lf and then shift P to 6° on the 

 scale. Pull the card forward again, carry T to the top of y 2 , 

 and then shift P to 12°. Continue this procedure right on 

 to the end of the curve. 



P will then have come back to its starting-point on the 

 scale, after describing on the card a certain curve or stepped 

 polygon. The area of the polygon is 



y x (sin 6° - sin 0°), 



+ y 2 (sin 12°- sin 6°), 



+y z (sin 18°- sin 12°), 



+ 



+y 4 (sin 360° -sin 354°), 

 Phil. Mag. S. 5. Vol. 39. No. 239. April 1895. 



2C 



