THE SUSPENDED PLANE. 13 



The suspended plane (plate IV) consists of a thin brass plane one foot square, 

 weighing two pounds, hung vertically by a spring from a surrounding frame. 

 Eight delicate friction rollers AA', BB' enable the plane to move freely along 

 the frame, but prevent any twisting or lateral motion, the use of the guide-frame 

 being to prevent the plane from so " flouncing " under irregular air currents that 

 its pull cannot be measured. The guide-frame carrying the plane turns symmet- 

 rically about an axis, CC', so that the gravity-moment about the axis is simply 

 the weight of the plane on a lever arm measured from its center. The axis 

 CC' rests upon a standard which is placed upon the whirling arm. A pencil, P, 

 attached to the plane is pressed by a spring against a registering card at the side 

 of the plane and perpendicular to it. The card contains a graduated arc whose 

 center is at C and whose zero angle is under the pencil point at the vertical 

 position of the plane. The distance of the trace from the center C registers 

 the extension of the spring. 



When the plane is at rest the extension of the spring measures the weight 

 of the plane. When the plane is driven forward horizontally the pressure of 

 the wind on the plane inclines it to an angle with the vertical, and the higher 

 the speed the more it is inclined. For any position of equilibrium there is 

 neither upward nor downward pressure on the guide-frame, and the whole 

 resulting force acting on the plane, both that of gravity and that arising from the 

 wind of advance, is borne by the spring. 



The apparatus being mounted at the end of the arm of the large whirling- 

 table and being still, the weight of the plane is registered by an extension of 

 the suspending spi'ing corresponding to two pounds. Next, lateral motion being- 

 given (from the whirling table) and the plane being not only suspended but 

 dragged forward, the spring is seen not to be extended further, but to contract, 

 and to contract the more as the speed increases. The drawing contains a copy 

 of the trace made by the pencil upon the recording sheet, showing how the 

 spring contracts with the increasing angles of the plane with the vertical, where 

 these angles correspond to increasing velocities of translation, or, we may almost 

 say, to increasing speeds of flight. The experiment also calls attention to the 

 fundamental circumstance that in the horizontal flight of an aeroplane increasing 

 speeds are necessarily accompanied by diminishing angles of the plane with the 

 horizontal. 



The experiment may perhaps be held to be superfluous, since the principle 

 involved, that the pressure of a fluid is always normal to a surfiice moving in it, 

 is already well known ; but we must distinguish between the principle and its 

 application. Though when attention is called to it, the latter is seen to be so 

 immediate a consequence of the principle as to appear almost self-evident, I must 

 still call the application "unfamiliar" since, as will be seen, it indicates the way 



