EESEAECHES IN SOUND. 487 



In arranging tlie signal the bell and luacliinery mnst be placed as low as possible, 

 as the sound is heard mucli more plainly on the water when the bell is near its sur- 

 face, and also as the machinery, when thus situated, is steadier and more readily ac- 

 cessible. 



Particulars as to the siren. — The boiler of a second-class apparatus is 12 feet long, 42 

 inches in diameter, and has 300 feet heating-surface. The dome is 2 feet in diameter 

 and 3 feet high. 



The cylinder of the engine is 4 inches in diameter and 6 inches stroke. The prolon- 

 gation of the piston-rod forms the plunger of the feed-pump. The main shaft carries 

 three pulleys, the larger driving the siren-spindle; the second, the worm and screw 

 gear ; and the third, the governor. 



In the worm-gear the wheel makes two revolutions per minute, and is provided with 

 a cam, which, acting on a lever, opens the valve, admitting steam through the siren- 

 disks. The cam has such a length as to hold the A'alve open for about seven seconds. 

 A counter-weight closes the valve as soon as the lever is released by the cam. 



The siren itself consists of a cylindrical steam-chest, closed at one end by a perfor- 

 ated brass plate. The iJerforations are twelve in number, equidistant from each other, 

 and arranged on the circumference of a circle, whose center is in the axis of the cyl- 

 inder. The other end is closed by a cast-iron head. The heads are connected by a 

 brass pipe, through which the sjiindle j)asses. 



The perforated head is covered on the exterior by a brass disk, attached to the 

 spindle, having twelve rectangular notches corresponding to the apertures on the for- 

 mer, and so arranged that by its revolution these ax)ertnres are simultaneously opened 

 and closed. The spindle is driven by a belt from the large pulley on the main shaft. 

 This shaft makes 180 revolutions per minute ; the spindle, 1,620 ; and as there are 12 

 apertures in the disks, from each there will issue jets of steam at the rate of 19,440 

 per minute. The sound produced by these impulses may be rendered more or less 

 acute by increasing or diminishing the velocity of revolution. 



The valve and valve-seat are disks similar to those already described, having how- 

 ever four openings instead of twelve. The valve revolves on the brass tube inclosing 

 4he siren-spindle, and is worked by a bevel gear. The trumpet is of cast-iron. 



The DaboU trumpet. — The apparatus used in the foregoing experiments is a second- 

 class trumpet, operated by an Ericsson caloric-engine. The air-pump is single-acMng. 

 Its cylinder is 12" in diameter by 12" stroke. The engine makes forty strokes per min- 

 ute. Tliere is a screw-thread raised on the main shaft, which, acting on a wheel, 

 drives a bevel gear, giving motion to a cam-wheel. The latter makes one revolution 

 in two ndnutes, and is furnished with three equidistant cams. These cams, pressing 

 on the valve-lever, throw the valve open once in forty seconds, admitting the com- 

 pressed air through the reed-chest into the trumpet. 



Ttie quantity of air forced into the tank should be in excess of that needed for the 

 trumpet, the surplus being allowed to escape through a delicate safety-valve. This is 

 necessary to i^rovide against a deficiency in case of leakage, and also to allow the 

 pressure of air to be regulated to accoaumodate the reed. Each reed requiring a differ- 

 ent pressure, it is necessary to alter the pressure of the valve-sj)ring whenever a reed 

 is changed. 



The iirst-class trumpet differs only in size from that described. 



The caloric-engine for the lirst class has a 30" cylinder. The air-pump is IG-V' by 

 15" stroke. 



The steam-ivhistle. — The boiler of this machine is that of the siren. On the forward 

 part of the boiler the bed-plate of a small engine is secured by two cast-iron brackets. 

 The cylinder of this engine is 4" by 9". The fly-wheel shaft carries an eccentric, 

 which, acting through a rod and jiawl on a ratchet-wheel, gives the requu-ed motion 

 to the cam-wheel shaft. 



The cam-wheel, which makes one revolution per minute, is provided with one or 

 more cams, depending on the number of blasts to be given in a minute ; the length of 

 the blast being regulated by that of the cams. 



The valve for admitting the steam into the whistle is a balance-valve, the diame- 

 ters of the two disks being respectively '^\" and 2J", which difference is sufficient to 

 cause the pressure of steam to close the valve tight without requiring too great a force 

 to open it. The valve is worked by a stem attached to the rocker-shaft at the lower 

 part of the steam -pipe. This shaft passes thi-ough a stuffing-box in the steam-pipe, 

 and is provided with a collar which the pressure of the steam forces against the inte- 

 rior boss on the pipe, thus making the joint steam-tight. The exterior arm en this 

 rocker-shaft, as well as that on the engine, is perforated in such a manner as to allow 

 the throw of the valve to be adjusted. 



