NO. 3 EUROPEAN AERONAUTICAL LABORATORIES ZAHM I3 



was employed by Mr. Mattullath and myself in our wind-tunnel 

 constructed in 1901 ; was disclosed to many others then; and shortly 

 thereafter was described in public prints. 



The true function of the " diftusor," or expandin^^ exit, seems to 

 be to prevent turbulence, and thus to promote economy of flow, rather 

 than to increase the pressure of the stream before it reaches the 

 fan, as taught by Eiffel. In other words, the economy of circulation 

 can be achieved by placing the screw at a narrower part of the exit 

 cone, if the pitch of the blades be properly adapted to the stream at 

 that section. But Eiffel's present arrangement presents structural 

 advantages. 



The circulation in the large tunnel is maintained by a Rateau screw 

 suction ventilator with helicoidal blades. The screw is driven by 

 a fifty-horse electric motor, which is found sufficient to maintain a 

 constant flow at any desired speed up to 2)- uieters per second, or say 

 up to 70 miles an hour. This is a notable result, since the air stream 

 at its swiftest section measures two meters in diameter. Though 

 the motor takes its current from the public mains, it requires little 

 adjustment of the rheostat to maintain steady speed for the time of 

 an observation, though it may vary in longer periods. 



The air velocity in Eift'el's tunnel seems to be satisfactory while 

 used for engineering studies rather than for exact researches in 

 physics. The velocity at all points of a cross-section is uniform 

 in magnitude to within two per cent, and varies but little in direction. 

 A fine silk thread, however, moored in the current, plays a trifle to 

 and fro in both the horizontal and the vertical direction. The current 

 velocity also fluctuates in time, say i to 2 per cent. 



The velocity is determined, as in the English and other laboratories, 

 from the pressure difference between the vacuum chamber and the 

 large room enclosing the tunnel. This pressure difference is meas- 

 ured with a Shultze manometer, or inclined tube containing alcohol 

 and provided with a graduated scale. In ordinary practice the end 

 of the alcohol column plays several per cent above and below a mean 

 reading, but can easily be located on the scale to within 4 per cent 

 by a capable observer. This means that the velocity can be deter- 

 mined truly to within two per cent. 



For convenience, in the determination of the wind effect on the 

 various kinds of models, Eiffel places his measuring instruments on 

 a platform, or bridge, spanning the vacuum room, and supported on 

 either side by wheels resting on iron rails secured to the walls, so 

 as to be moved aside when desired. Sometimes also the models are 



