236 Proceedings of the Royal Society of Edinburgh. [Sess. 
Table I. 
0° 15 30 45 60 75 90 105 120 135 150 165 
180 
360 345 330 315 300 285 270 255 240 225 210 195 
e I 0 -0027 -0101 '0196 '0284 '0341 '0352 '0317 '0247 '0160 '0079 '0021 0 
The above results have been graphed in fig. 4, which shows a curve 
of two almost symmetrical waves. Hence it is apparent that if two 
A is the original curve. 
B is the same curve retarded 90° in phase. 
C is the sum of the two. 
“ crank-and-wire ” units could be arranged to work together , one being 
90° in phase in advance of the other, the hills and hollows of the two 
would very nearly cancel each other , and so eliminate the error. 
The special design of the apparatus by which this is accomplished will 
be understood from fig. 5. Imagine a 
cord fixed at B to be let down under 
the crank pin P and carried up over 
the fixed pin A, the end D being con- 
strained to move in the straight line AD. 
BCP is in itself a complete “ crank - 
and-wire ” unit, as also is ACP, and the 
phase difference between these units can be made 90° by taking BCA 
equal to 90°. 
