ACOUSTICS. 77 



a foot ; and when the number of segments is as many as 

 sixteen or twenty, they can all be seen and counted by 

 a large number of persons at a time. If the string 

 a, b, is twice as long, and may reach back to a, the 

 free end may be held in the left hand while the fork is 

 struck with the right. It will then be very easy to vary 

 the tension of the cord while it is vibrating, and the 

 segments can be made to change through its whole 

 series of one, two, three, four, etc. The various forms 

 and motions of the cord may be shown to still better 

 advantage, by making a strong beam of light from the 

 porte lumiere or lantern to fall upon it in the direction 

 of its length. 



Crova's apparatus consists of disks of glass about 

 fifteen inches in diameter, which may be made to turn 

 upon a suitable rotator. These disks are at first painted 

 black, and then curves of various forms are traced 

 through the paint to the glass. The upper part of the 

 disk is projected in the ordinary way, and then if it be 

 rotated, the lines which are drawn upon it will appear 

 to move or to be quiescent, according as they are con- 

 centric, eccentric, or some other form. If a diaphragm 

 with a slit in it, long enough to reach across all the 

 lines which are drawn upon the disk, be placed behind 

 it, a series of dots will appear upon the screen, which 

 will change their positions as the disk turns round. 



With properly drawn curves the various wave-motions 

 in air in organ-pipes, reflection of sound-waves, nodes, 

 interference, and so forth, as well as the transverse vi- 

 brations in light-waves, may be well shown. 



AN ATTACHMENT TO THE WHIRLING TABLE FOR PRO- 

 JECTING LISSAJOU'S CURVES. 



Two posts p and p' are made fast to the frame upon 

 the opposite sides of the inertia plate a. A small 



