310 PROCEEDINGS OF THE AMERICAN ACADEMY 



the power required to produce it, and is, therefore, proportioned to the 

 magnitude of the receiver when other circumstances are the same ; 

 whereas its statical power or its power to sustain a head of water is 

 wholly independent of the magnitude of the receiver, and proportioned 

 solely to the tension of the air within it. In all these cases, there is a 

 striking difference between the operations of using the statical and 

 dynamical effects, which deserves the most careful consideration, be- 

 cause it is essential and characteristic. The statical effect may be 

 used for any length of time without being impaired, and the reason is 

 obvious ; it manifests itself in a state of rest, when there is no change 

 of condition. The dynamical, on the contrary, can be used once and 

 but once. The one pound can balance the hundred pounds as long as 

 the materials of the pulley and lever will endure ; a compressed spring 

 may sustain its weight, or the expanded air its head of water, as long 

 as we choose, without any diminution of effect. But when work is to 

 be done, a change to be effected, a weight to be raised, a velocity to 

 be produced, the result can only be obtained by a corresponding change 

 in the opposite direction, an undoing of work, a falling of a weight, a 

 consumption of power once and for ever. In the present case, in which 

 the object is to obstruct or divert the motion of the vv'ind in such a way 

 that part of its velocity may be communicated to the air in the chimney, 

 and thus produce a current, the amount of this communication and trans- 

 fer of velocity cannot be measured when it does not take place, — when, 

 on the contrary, the mouth of the chimney is entirely stopped up, so that 

 it is impossible to produce any current within it. It would be just as 

 proper to weigh a water-wheel by the weight which will just reduce it 

 to a state of rest, instead of that smaller weight which reduces it to its 

 usual working velocity, and which is universally adopted by experi- 

 enced engineers as the correct measure of the power of the wheel. 

 It should also be borne in mind, that there are resistances offered to 

 air in motion by the tube through which it passes. These resistances 

 are not constant ; they increase as the perimeter and length of the tube 

 directly, and also as the square of the velocity ; these, it is obvious, 

 cannot be measured where they do not exist. 



" The plan, therefore, which has been adopted in these experi- 

 ments, is to measure directly the velocity of the current produced, and 

 it will not be surprising, after what has preceded, if some striking dif- 

 ferences should be observed between the results thus obtained and 

 those derived from any statical measure. 



