28 EXPERIMENTS IN AERODYNAMICS. 



the quadnxnt contacts of the turn-table, the currents for the moment being cut 

 off from the quadrant contacts and sent through the Plane- Dropper. 



The dimensions and weight of the principal parts of the apparatus are as 

 follows : 



Length of brass tul)e 160 centimeters. 



Length of aluminum falHng-piece 25 " 



Length of buffers 5 " 



Actual distance of fall (between contacts) 122 " 



Distance of center of brass frame ami falling-i^iece from center of 



turn-table, when mounted 981 " 



AW-ight of falling-piece 350 grammes. 



The planes are made of varnished pine about 2imm. thick, and stiffened on 

 one edge with an aluminum strip. 



Five different pairs were used, having the following dimensions and 

 weights : 



(1) Two planes, each *6 x 12 in. (15.2 x 30.5 cm.) ; weight of pair, 123 grammes. 



(2) " " "8x9 in. (22.9 x 20.3 cm.) ; " " 115 



(3) " " " 12 X 6 in. (30.5 x 15.2 cm.) ; " " 114 " 



(4) " " " 18 X 4 in. (45.7 x 10.2 cm.) ; " " 114 



(5) " " " 15 X 4 in. (38.1 x 10.2 cm.) ; " " 118 " 



Each pair of planes, therefore, e.xcept the last, has an area of one square 

 foot, and weighs, with the aluminum falling-piece, appro.ximately one pound. 



It may be desirable to add that this instrument was constructed with special 

 pains in all the circumstances of its mechanical e.xecution, the veiy light falling- 

 piece, for instance, moving on its friction wheels so readily that it was not 

 possible to hold the rod in the hands sufficiently horizontal to keep the " falling- 

 piece " from moving to one end or the other, like the bubble of a level held in 

 the same manner. 



Preliminary e.xperiments were made to determine the effects of friction on 

 the time of fall, when the Plane-Dropper is in rapid horizontal motion, by drop- 

 ping the aluminum falling-piece without planes attached, and it was found that 

 under these circumstances the time of fall is not sensibly greater when in rapid 

 motion than when at rest. As a further test, the j^lanes were then attached to 

 the falling-piece in a vertical position, that is, so as to present their entire surface 

 to the wind of rotation, and thus to produce a friction very much greater than any 

 occurring in the subsequent e.xperiments ; but the time of fall was not increased 

 to any notable degree. The effect of friction and other instrumental errors are 

 shown thus, and by considerations already presented, to be negligible in com- 

 parison Avith the irregularities inevitably introduced by irregular air currents 



* First measurement refers to advancing edge. 



