8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



Such gauges are equally useful for measuring the difference 

 between some standard pressure and the actual pressure at various 

 points on the surface of a model, or elsewhere. Thus one arm of 

 the U tube may be connected with the internal surface of the tunnel 

 while the other arm is connected successively to various points on 

 the surface of a model. The difference between the standard wall 

 pressure and that at each point of the model surface may then be 

 plotted giving a diagram of surface pressure distribution all over 

 the model. 



The wind balance. — Besides the pressure distribution and resultant 

 pressure on models, it is desirable to determine also the total wind 

 force, which is composed of both the pressure and the friction of 

 the air. To this end the English experimentalists use a bell-crank 

 balance which is a modification of the type devised and used in my 

 laboratory, and now employed also by Eiffel for the accurate meas- 

 urement of small wind forces. The English balance consists of tw^o 

 horizontal weighing arms, one parallel to the tunnel, the other 

 perpendicular, attached to a round vertical tube, or arm. supported 

 at its center on a conical pivot just beneath the tunnel floor. The 

 vertical arm of the bell-crank balance has its upper half extending 

 through the tunnel floor up to the center of the current and is duly 

 shielded by a stream-line encasing sheath, while its lower half extends 

 downward from the pivot and dips into a pail of oil intended to 

 damp the oscillations. The upper half supports at its extremity 

 the wind model and near the pivot has a graduated joint, so that it 

 can be rotated about its own axis, and thus orient the model as 

 desired. Sliding weights on the two weighing arms are made to 

 measure the components of wind force parallel to the flow and per- 

 pendicular thereto. If the wind force tends to rotate the balance 

 about the vertical axis, this tendency, or wind moment on the model, 

 is determined by observing what horizontal restraining force must 

 be applied to one of the horizontal arms to prevent such turning. 

 Thus the balance may be used to measure lift, drift and center of 

 pressure. There are numerous ingenious and important details, 

 such as those for studying stability coefficients, w-hich can best be 

 obtained from the British Aeronautical Committee's technical report 

 for 1 91 3, or from the working drawings which the laboratory has 

 furnished the Smithsonian Institution. Though this aerodynamic 

 balance is accurate and moderately convenient, I am of the opinion 

 that several new types can be devised which shall ])e e(|ually precise 

 and ])robably more expeditious, recjuiring less adjustment at each 



