8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



equal angles to right and left of mid position differed 0.5 per cent from 

 the mean of all readings. The extreme error of one observation, 

 including the error of 0.5 per cent due to lack of sensitivity of the 

 balance and the personal error of the observer, may then be as great 

 as I per cent in case the two errors are cumulative. 



It is not considered practicable to obtain a closer setting with the 

 methods of alignment employed. 



Setting Arms of Balance 



Knowing the true direction of the wind, it is necessary to set the 

 horizontal arms of the balance parallel and perpendicular to this 

 direction. To do this the floating part of the balance was rotated by 

 an adjustment provided by the design until the force recorded on the 

 " drift " arm (the resistance) was equal for equal angles of the plate 

 to right and left of the wind direction. The final setting indicates a 

 remaining error of .2 per cent. 



After making the slight adjustment required here the error in the 

 transverse force measurement was not found to be increased. 



Measurement of x\ir Velocity 



The velocity of flow in the wind tunnel is measured by a pressure 

 tube anemometer commonly called a double Pitot tube. 



Our laboratory standard is a double Pitot tube presented by the 

 director of the National Physical Laboratory, England. This tube 

 was compared with the National Physical Laboratory standard which 

 had been calibrated on a whirling arm by F. H. Bramwell.^ Its con- 

 stant had been determined to be unity to a precision of o.i per cent. 

 Our tube was compared with it by a method allowing a precision of 

 0.25 per cent. A discrepancy of about 0.25 per cent was found. Its 

 readings may then be taken as correct to this degree of precision. In 

 all cases a uniform rectilinear current is implied. 



The Pitot tube, in common with all anemometers, has the disad- 

 vantage of obstructing the channel, and where models are to be tested 

 the channel should be kept entirely clear. The expedient of using a 

 side hole in the channel is due to M. Eififel.' 



In a channel of uniform section, air is forced to flow practically 

 parallel to the axis of the channel. Hence stream lines are all parallel 

 and across any section, taken normal to the channel axis, there should 

 be no component of velocity at any point. This statement is of course 

 true only for a steady, uniform flow free from turbulence. The 



^Technical Report of the Advisory Committee for Aeronautics, London, 

 1912-13. 

 'La Resistance de I'Air et Aviation, Paris, 1912. 



