1874] WRITINGS OF JOSEPH HENRY. 397 



Another element of importance in the construction of 

 these instruments is the volume of sound. To illustrate this 

 it may be mentioned that a harpsichord-wire stretched be- 

 tween two strings of India rubber when made to vibrate by 

 means of a fiddle-bow, gives scarcely any appreciable sound- 

 We attribute this to the want of quantity in the aerial wave; 

 for if the same wire be stretched over a sounding-board hav- 

 ing a wide area the effect will be a comparatively loud sound 

 but of less duration with a given impulse. It was therefore 

 suggested that the width of the reed in the DaboU trumpet, 

 the form and size of the hole in the disk of the siren, and the 

 circumference of the vibrating sheet of air issuing from the 

 circular orifice of the whistle, would affect the power of the 

 sound. The only means of testing this suggestion is by using 

 reeds of different widths, sirens with disks of different-shaped 

 openings, and whistles of different diameters. In conformity 

 with this view Mr. Brown has made a series of empirical 

 experiments with openings of different forms, which have 

 greatly improved the operation of the siren, while Mr. Wilcox 

 has experimented on several forms of reeds, of which the 

 following is the result: 



The best reed obtained was 2|- inches wide, 8 inches long in 

 the vibrating part, f inch thick at the butt, and ^ inch thick 

 at the free end. This sounded at a pressure of from 20 to 30 

 pounds. The thinner reeds gave a sound at a less pressure, 

 from 5 to 10 pounds, the thicker at from 20 to 30 pounds. A 

 reed 8| inches long in the vibrating part, 1 inch thick at the 

 butt, f inch thick at the end, and 3 inches wide, did not begin 

 to sound until a pressure of 80 pounds was reached, when it 

 gave a sound of a dull character. Another reed of the same 

 width, § inch thick at the butt, and yV iiich at the end, and 

 same length, gave a sound at 75 pounds pressure, but still dull 

 and of little penetrating power. These reeds were evidently 

 too heavy in proportion to their elasticity. These were made 

 without the addition of a trumpet, and therefore to produce 

 the best result when used with a trumpet the latter must be 

 increased or diminished in length until its natural vibra- 

 tions are in harmony with those of the former, as will be 



