4 SCIENCE 



and many others have been the subject of 

 long and difficult scientific examination. 



At the National Physical Laboratory there 

 are five wind channels continually in use to 

 test on models all the various factors on which 

 the aerodynamic efficiency of a machine 

 depends. Two of these channels are 7 feet 

 in diameter and nearly 80 feet in length ; 

 in one, wind speeds up to 60 miles an hour 

 can be obtained. The model is attached to 

 a specially designed balance or dynamometer 

 and the forces it experiences in various 

 positions relative to the wind are measured; 

 from these data the behaviour of the machine 

 in flight is determined. Here Mr Bairstow 

 and his colleagues have worked out the 

 practical conditions of stability of motion and 

 determined by many ingenious devices the 

 constants which occur in the theory. That 

 theory was first given in a general form by 



