OF ARTS AND SCIENCES. 309 



we obtain a velocity of 124.32 inches, or 10.36 feet, per second, — 

 7.06 miles per hour. This, therefore, may be assumed as a near ap- 

 proximation to the velocity of the blast, when not otherwise mentioned. 

 " The velocity of the induced current being the true measure of the 

 practical value of different forms of ventilating apparatus, it becomes 

 necessary to ascertain this value as accurately as possible. The in- 

 convenience attending measurements in which time is involved as one 

 of the elements, and also, probably, the difficulty of determining the 

 instant when a current has passed through a certain space, have led to 

 the adoption of other means, by which the velocity of the current is 

 not directly measured, but inferred. The mode which has been re- 

 peatedly adopted, of measuring the efficiency of a ventilator by its 

 power of sustaining a weighted flap or valve, or a head of water, or 

 by some other statical effect, is decidedly objectionable. Such a meas- 

 ure gives the correct value of the initial force or tendency to establish 

 a current in a chimney in which there is no actual movement ; but it 

 does not indicate the velocity of the current which will be the final 

 result of the action of the ventilator, nor is it any measure of this 

 final velocity when ventilators of different construction are compared 

 together. Mechanics and engineers are familiar with the difference 

 between the statical and dynamical effects of a force. They are aware 

 that the former may be greatly increased by the mechanical powers, 

 so that, through the medium of a pulley or a lever, a single pound may 

 be made to sustain and raise a hundred times its own weight. But the 

 dynamical effect is not correspondingly increased, for in order to raise 

 one hundred pounds through the height of a foot, the one pound must 

 in all cases fall one hundred feet ; so that the loss of height precisely 

 balances the gain in weight. In the same way, the dynamical effect 

 of different springs is not to be measured by their strength alone ; it is 

 not simply dependent upon the amount of weight which they will sus- 

 tain, but equally upon their length, or rather upon the distance through 

 which they move in restoring themselves to equilibrium. The archer's 

 bow is a good instance of this assertion, which any one can try for 

 himself, and he will find, that, with a given exertion of strength, he is 

 able to throw the arrow farthest and highest with that long bow of 

 which he can draw the string to his full arm's length, and not with the 

 strong bow which he can hardly move. But an example more nearly 

 allied to the case under consideration is derived from the air-pump, in 

 which the dynamical value of any amount of exhaustion is equal to 



