VT^TP 



LECTURE XXIX. 



calculated to correct the difference of pressure arising from the greater or less 

 immersion of the cylinder. (Plate XXIV. Fig. 332.) 



A shower of water, or even an irregular stream, being conveyed through 

 a descending pipe, plunged into the water of a reservoir, a large quantity 

 of air is carried down with the water, and rises to the upper part of an in- 

 verted vessel which surrounds the pipe, whence it may be conveyed through 

 another pipe, in a rapid stream, for any required purpose; and the water es- 

 capes at the bottom of the air vessel into the general reservou-, from the 

 surface of which it runs off. The quantity of air supplied by these shower 

 bellows is, however, small. (Plate XXIV. Fig. 333.) 



The velocity of the blast produced by any pressure, forcing the air through 

 a pipe of moderate dimensions, may readily be determined from the height of 

 a column of air equivalent to the pressure. Thus, if the hydraulic bellows 

 were worked with a constant pressure of 4 feet of water, the velocity would 

 correspond to a height of about 3300 feet, and the air would move through a 

 space of about 460 feet in a second. But in this calculation no allowance is 

 made for any of the causes which diminish in all cases the discharge of fluids, 

 and the velocity actually observed is only five eighths as great as that which cor- 

 responds to the height; that is, in the example here given, £85 feet in a 

 second, when the air escapes through a small orifice; but when it moves in 

 a pipe, about three fourths, or 345 feet. If the pipe were of considerable 

 length, there would also be a diminution of velocity on account of friction. 

 In some bellows actually employed, a pressure equivalent to 9 feet of water is 

 applied, and in this case the velocity must be about 500 feet in a second. 



Bellows may be used for the ventilation of a mine, either by forcing air 

 into it, or by drawing it out through a pipe connected with the valve. 

 The wind may also be received by the mouth of a tube a little conical, and 

 may be made to cause a current where it is conveyed; such an instrument is 

 sometimes called a windsail, or a horse head. It has been proposed to draw 

 the air up through a pipe by the lateral, friction of a current of air received 

 by such a funnel, but the effect would probably be too small to be of much 

 practical utility. 



