INDUSTRIAL PROGRESS DURING THE YEAR 1875. xlix 



under equal pressures, as is common, but by measuring the 

 pressures, the volumes being made equal. It is simple in 

 construction, satisfactory in operation, and accurate in its re- 

 sults. 



ACOUSTICS. 



In Acoustics, Lissajous has described in the Bulletin de la 

 Societe d' * Encouragement an elaborate machine for tracing 

 mechanically the curves which represent the composition of 

 vibratory movements, constructed by Froment. The driv- 

 ing-shaft carries toothed wheels, gradually increasing in size 

 from right to left. Upon the pinions driven by these, which 

 are arranged in pairs, are eccentrics, which by means of con- 

 necting rods o-ive a differential to and fro motion to an arm 

 transverse to their direction. To the centres of two contigu- 

 ous arms two other connecting rods are attached, which move 

 a transverse arm of the second order, and similarly an arm 

 of the third order is thus moved, which carries the style. 

 The motion of the style is therefore the algebraic sum and 

 resultant of the motion of the eight driving-wheels, and the 

 curves it describes may be exceedingly complicated. 



Schuller has contrived an apparatus by which Lissajous's 

 figures may be readily produced on the screen. It consists 

 of two pendulums, adjustable by sliding weights, carrying 

 mirrors, each movable on a horizontal axis, at their upper 

 ends. The planes of vibration may be parallel or perpendic- 

 ular, at will. The same physicist has devised a modification 

 of the common form of this experiment with tuning-forks. 

 Instead of having a mirror on the extremity of a prong of 

 each fork, he places the two forks with their four prongs in 

 the same plane, one of the forks being vertical, and four or 

 five inches in advance of the other, which is horizontal. The 

 lower prong of the horizontal fork carries a screen with a 

 small hole in it. The second fork carries on one of its prongs 

 a small lens of short focus. The small opening in the screen 

 is strongly illuminated by sunlight concentrated on it by a 

 lens ; an image of this is formed on a distant screen by means 

 of the lens on the second fork. When the first fork is vibrat- 

 ing, a vertical line of light will appear; when the second is 

 in motion, the line will be horizontal ; when both are in ac- 

 tion, the Lissajous curve corresponding to their rate will be 

 2;iven. The figures are much larger made in this way. 



