14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



supported on a frame which can be wheeled along the floor. Thus 

 apparatus can be adjusted outside the tunnel, quickly run into place, 

 and again removed without dismantling. This is a unique advantage 

 of Eififel's arrangement. The main apparatus so employed are the 

 aerodynamic balances, the propeller tester, and the instruments 

 respectively for tnnding the dislrilmtion of pressure and the magni- 

 tude and line of action of the total wind force. 



Of the tzvo balances the simple bell-crank one for the precise 

 measurement of smaller forces has been sufficiently explained as to 

 principle in describing the English laboratory. The large aerody- 

 namic balance, invented by Eiffel himself for determining the lift 

 and drift of the whole wind force, and its line of action, is elaborate 

 in theory, structure, and practical operation, and is well explained in 

 Eiffel's book, " The Resistance of the Air and Aviation." It is not 

 sensitive enough for measuring the smaller forces on inclined planes 

 and on small models. 



The propeller tester is elegantly simple in design and operation. 

 A vertical electric motor, mounted on the bridge above the tunnel, 

 and having its shaft extended down through a wind shield to the 

 center of the air stream, there engages, through bevel gearing, with 

 the horizontal shaft of the model propeller. The shafting of the 

 armature and the propeller are encased in a sheathing which also 

 contains the bearings, and transmits the propeller thrust and torque 

 to the base of the motor. The motor, in turn, is so mounted on 

 pivots and hydraulic gauges as to measure the thrust and torque 

 without material displacement At the same time the motor s])eed 

 is indicated by a tachometer attached to the upper end of the arma- 

 ture shaft. The wattmeter method, hov/ever, has lately replaced 

 the direct method of measuring propeller torque. 



The apparatus for lueasuriiiSi;; the distribution of air pressure over 

 the surface of models has long been used by others, and in principle 

 is like that employed in the English laboratory, and hitherto described 

 in this report. The instrument for finding directly the line of the 

 resultant air force, or " center of pressure," on a model surface is 

 also an old contrivance, and need not be explained here. It is fully 

 described in Eiffel's .book.' 



^ It may be noted, however, that Eiffel's and the English method of allowing 

 a model to rotate about a vertical axis by supporting it on a step bearing is not 

 very dehcate, even when a jewel step is used. A more accurate way is to 

 suspend the body from a wire, or float it on a liquid. The writer, in 1901, 

 discarded the jewel pivot and supported his models on a fine steel wire, an oil 

 damper being provided to deaden oscillations. With a float no damper is 

 needed. 



