ON PNEUMATIC MACHINES. iJI'i 



A corn fan is turned by the hand, or by machinery ; its simplest operation 

 is to cause a portion of air to revolve with it, and to create a wind in the di- 

 rection of its circumference. But when a small fan is made to revolve with 

 great rapidity, as in Papin's Hessian bellows, the centrifugal force causes 

 the air admitted at the centre to rush towards the circumference, and to pass 

 with great velocity through a pipe inserted there. The common ventilator 

 placed in windows, which revolves in the same manner as a smoke jack, in 

 consequence of the impulse of a current of air, serves only to retard a little 

 the entrance of that current, to disperse it in some measure in different di- 

 rections, and to prevent any sudden increase of the intensity of the draught; 

 but it has little or no power of acting on the air, so as to prevent the decrease 

 of the velocity of the current. (Plate XXIV. Fig. 334.) 



The operation of heat affords us also a very effectual mode of ventilation. 

 Its action upon air at common tempeiatures occasions an expansion of 

 about -j^TT for every degree that Fahrenheit's thermometer is raised; the air 

 becomes in the same proportion lighter, and the fluid below it is consequently 

 relieved from a part of its weight : the pressure of the surrounding atmosphere, 

 therefore, preponderates, and the lighter column is forced upwards. When the 

 shaft of amine communicates with the external air at two different heights, 

 there is generally a sufficient ventilation from the difference of the tempera- 

 tures of the air in the shaft, and of the surrounding atmosphere: for the 

 temperature of the earth is nearly invariable, it therefore causes the air in 

 the shaft to be warmer in winter than the external air, and colder in sum- 

 mer; so that there is a current upwards in winter, and downwards in sum- 

 mer; and in the more temperate seasons, the alternations take place 

 in the course of the day and night. For a similar reason, there is 

 often a current down a common chimney in summer ; but when the fire 

 is burning, the whole air of the chimney is heated, and ascends the more 

 rapidly as the height is greater. It would be easy, from the principles 

 of hydraulics, if the length of the chimney, and the mean temperature 

 of the air in it were given, to calculate the velocity of the draught: thus, 

 if the height of the chimney were 50 feet, and the air contained in it 

 10 degrees hotter than the external air, the expansion would be one fif- 

 tieth, and the pressure of the whole column being diminished one fif- 

 tieth, the difference would be equivalent to a column of one foot in height, 



VOL, I. " y y 



