39 



velocity increasing as the square of the time ; there- 

 fore, the relative velocities are, as the square roots 

 of the heights. — (Hood on Ventilation, p. 31.) 



i • 

 I i 



Now, in the pipe sketched above, the total height 

 from the floor of the stove to the point of final escape 

 of the heated air, is the height of the syphon. The 

 force of motion is the difference of weight between 

 the column of heated air and that of a column of the 

 external air of the same height. Now air expands 

 when heated 1 -480th of its bulk for each degree of 

 Fahrenheit's thermometer, and the velocity of motion 

 is equal to the additional height which a given weight 

 of heated air must have in order to balance the same 

 weight of cold air ; therefore, the higher the tube in 

 the chimney, and the hotter the air is kept in it, the 

 greater the velocity of ventilation. 



The passage of air may be regulated by a slide to 

 pass through a cut in the pipe just within the house, 



