■208 Proceedings of Poyal Society of Edinburgh. [sess. 
the author to add another to their number. In the first place, it 
was desired to ascertain whether results of a high degree of accuracy 
could not be obtained from an apparatus of quite inexpensive 
design ; and in the second, to construct a machine in which both 
the amplitude and periodic time of each of the simple harmonic 
constituents might be varied at will while the instrument is in 
motion. 
§ 1. General Description. 
The apparatus (see Plate, fig. 1) has a long narrow wooden 
base, from which spring uprights, carrying the horizontal axes 
of a succession of wheels arranged in a line, one after the other. 
The wheels are made of mahogany, with V-shaped grooves turned 
in their edges, and they are driven from a small electric motor 
by means of a single, endless leather belt of circular section. 
These wheels form the simple harmonic constituents of the 
machine : and they, or others geared to them, as will be explained 
later (see the section on change of period, and also that on 
change of amplitude), have each a pin fixed in the wheel at some 
distance from the centre, which stands out normally to the wheel, 
and has mounted on it a freely turning brass pulley with grooved 
edges. 
The base of the instrument is screwed down to a table from 
which uprights arise to carry a sort of light horizontal frame 
some three feet above. A length of jSTo. 38 silk-covered copper 
wire, with one of its ends fixed to this upper frame, is led vertically 
down and round one of the small eccentric pulleys just mentioned, 
and then returns up again to the frame (see fig. 7). Passing 
round a fixed pulley there, it once more descends, and passes 
round the brass pulley on the pin of another of the simple 
harmonic wheels, after which it returns back to the bracket ; and 
so on, until all the simple harmonic wheels have been included. 
Form of Harmonic Analyser,” G. V. Yule, Phil. Mag., vol. xxxix. p. 367, 
1895. “Harmonic Analyser,” Geo. H. Rowe, Electrical World and Engineer , 
p. 587, 25th March 1905. Three papers by Prof. Sir W. Thomson, Proc. 
Roy. Soc., pp. 266, 269, 271, 1876. “On an Integrating Machine having a 
New Kinematic Principle,” Prof. James Thomson, Proc. Roy. Soc., vol. xxiv. 
p. 262, 1876. “ On a New Harmonic Analyser,” Prof. 0. Henrici, Phil. 
Mag., vol. xxxviii. p. 110, 1894. “Ueber Instrumente zur harmonischen 
Analyse,” 0. Henrici, Catalogue Munich Mathematical Exhibition, 1892-3. 
