210 Proceedings of Royal Society of Edinburgh. [sess. 
has been found satisfactory : it allows the use of any period 
commensurable or incommensurable, and of the alteration of the 
period during motion. In fig. 2 will be seen a trace of a S.H.M. 
FiG. 1 . — It has been found better to slightly alter the arrangement 
sketched above , and make the guide pulley A act on the upper side 
of the belt. Cf. the later photograph on Plate, fig. 1. 
in which the periodic time gradually increases, the lever D 
having been slowly slid along the axis E by hand as the machine 
was running. 
§ 3. Description of the Mechanism which enables the Amplitude of a 
Constituent Harmonic to be varied during motion. 
The mechanism used to change the amplitude will be seen in 
the stereoscopic photograph, Plate, fig. 3. 
The further off vertical toothed wheel has rigidly attached to it 
a grooved wooden wheel round which passes the motor driving- 
belt. Motion is communicated to the nearer vertical toothed 
wheel through the intermediary of the “ crown ” wheel. The 
