AIR CURRENTS IN RELATION TO AVIATION 369 



records variation in direction shows a trace very similar in 

 character to the trace of the pressure tube anemograph which 

 records velocity. See the record of direction for Alnwick, fig. 14. 



Suppose that the surface current is something of the nature 

 of a vast stream of water flowing along a channel, what are the 

 changes in the direction and velocity? 



That there are frequent and irregular changes in velocity 

 and equally frequent and irregular changes in direction we 

 know. We want to find out how these are related (10), be- 

 cause if a change of velocity is accompanied by a change of 

 direction the lateral stability of the machine has to be considered 

 as well as the longitudinal. In order to examine this point 

 the Advisory Committee for Aeronautics has found means for 

 me to have an investigation carried out by Mr. J. S. Dines 



Fig. 19. — Vector diagrams of wind at 98 ft. for 1 min., 1 min., and 25 sec. respectively, 

 showing irregular fluctuations in direction and velocity. 



at Mr. W. H. Dines's establishment for the study of the upper 

 air, in connection with the Meteorological Office, at Pyrton 

 Hill (11). Mr. J. S. Dines has worked out the design of an 

 instrument by which direction and velocity of the wind at any 

 moment is recorded as a single point on a diagram and the 

 complete diagram, which we call a vector wind diagram, repre- 

 sents the sequence in the direction and velocity of the wind 

 during the period that the instrument is allowed to operate. 

 Naturally there are some instrumental difficulties. It is not 

 certain how far the record of a pressure tube anemograph in 

 a wind that is varying very rapidly is a strict record of the 

 velocity of the wind ; all the different parts of the apparatus 

 have momentum and momentum interferes with the fidelity of 

 the record. The same is more obviously the case with the 

 wind direction. A sudden gust may set the vane in motion 

 but how far it turns depends in part on its momentum. 



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