6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



slides freely on rollers inside the main vertical arm of the balance. 

 The lower end of this rod rests on one end of a horizontal arm having" 

 a knife edge and sliding weight. 



For special work on moments, the interior vertical rod is replaced 

 by another having a small bell crank device on its head which con- 

 verts a moment about the center of the model into a vertical force to 

 be measured as above (pi. 4) . 



In this way provision is made for the precise measurement of the 

 three forces and the three couples which the wind may impress on 

 any model held in any unsymmetrical position to the wind. 



The balance is fitted with suitable oil dash pots to damp oscillations, 

 and devices for limiting the degrees of freedom to simplify tests in 

 which only one or two quantities are to be measured. The balance 

 can be adjusted to tilt for 1/10,000 pound force on the model. In 

 general, the precision of measurements is not so good as the sensi- 

 tivity, and in the end is limited by the steadiness of the wind and the 

 skill of the observer. 



The weights and dimensions of the balance were verified by the 

 National Physical Laboratory, where also the torsion wires were 

 calibrated. 



For ordinary forces, weighings may be considered correct to 0.5 

 per cent. Naturally for very small forces, such as the rolling moment 

 caused by a small angle of yaw, the measurements cannot be so 

 precise. 



Alignment of Tunnel 



The axis of the wind tunnel was desired to be horizontal from the 

 honeycomb to the baffle plates in front of the propeller. To accom- 

 plish this an engineer's level was mounted on a platform, built on the 

 floor of the house, opposite the mouth of the tunnel, and sighted on 

 the intersection of diagonal threads placed at 6- foot intervals. By 

 this means the distance of the center line of the tunnel above or below 

 the horizontal line could be estimated to one-eighth of i inch. 



The tunnel beling low in the center, it was raised by wedges 

 until the reference marks coincided with the horizontal. This was 

 attained to within one-eighth of i inch in 6 feet of tunnel length. 

 The tunnel may, therefore, be said to have its axis horizontal to within 

 one-tenth degree. 



Alignment of Vertical Axes of Balance 



A concrete foundation having been built for the balance, the latter 

 was set in its approximate position. Three wedges were then 



