INSTRUMENTS FOE EXPERIMENTS ON SOUND. 151 



disc of cardboard pierced with a number of holes at regular 

 intervals. It is made to rotate in front of a small wind-chest 

 in which there are keys so that a stream of air can be directed 

 against any particular ring of holes. The form adopted by 

 Cagniard de la Tour merely makes these holes vertical, and 

 has a little pipe coming up. A still more perfect form is Dove's 

 polyphonic siren, which has been perfected by Helmholtz. 

 Here is Seebeck's siren. I make it rotate, and open the 

 little keys, and you hear the different tones. As I drive faster 

 the notes rise up sharper and sharper. It gives, however, a 

 very feeble and wretched note ; the object in the latter in- 

 strument has been to produce a more powerful one. This 

 double siren cannot be said to err on the side of lack of power. 

 It has two perforated rotating discs on one axis, each con- 

 neeted with wind-chests, which are covered by outside boxes, 

 so 'as to give more purity to the sound. The lower wind-chest 

 is fixed, but the upper can be rotated upon its axis. Each of 

 them contains four rows of oblique holes, and each of these 

 four rows of holes can be brought into operation by touching 

 a valve which draws in and out. These instruments require 

 very great wind pressure; we have now a pressure on the 

 bellows of more than one hundredweight ; equal to a column 

 of twenty-six inches of water. When this high-pressure 

 wind is allowed to pass through the oblique holes in the wind- 

 chest against the corresponding holes in the rotating disc, the 

 latter is soon set into rapid motion. At first a series of puffs 

 is heard, as the two sets of holes coincide and interfere, but 

 these soon merge into a continuous hum, and mount into a 

 note, which rises steadily in pitch as the rotation becomes 

 more rapid. An attached counter enables the speed to be 

 measured by means of a watch. If you examine the ratios 

 as I have written them down on the black board, you will 

 find that on the lower disc there is a row of eight holes, 

 outside that a row of ten, then a row of twelve, and then a 

 row of eighteen. These give us, if you take C for the lower 

 note, E, G, and D. On the upper disc there are 9, 12, 15, and 

 16, which give D, G, B, and the upper C. We can thus get 

 unison by taking the two G's ; an octave by taking two C's, 

 a 5th by taking C and G, and so we can work through the 

 various consonant intervals. I have not time to demonstrate 

 these facts fully, but I will consider one or two. Starting with 

 unison on the two discs, I rotate the handle moving the top 



