368 SCIENCE PROGRESS 



For fliers we have to deal mainly with the texture of the air 

 current. It is clear enough that the only condition of safety 

 with respect to a cliff eddy or the eddies due to other obstacles 

 of various kinds is that they must be avoided and localities for 

 alighting selected which are sufficiently far removed from such 

 occurrences. We turn our attention therefore to the gustiness 

 of ordinary winds. 



The gustiness must be for the flying-machine what a bumpy 

 road is for the cyclist or a bumpy sea for a small boat. In 

 consequence of the variation of velocity above mentioned, the 

 airman in a gusty current must be prepared for constant atten- 

 tion to the maintenance of his level against variations of lifting 

 force and to his longitudinal stability ; in so far as the gustiness 

 is dependent upon exposure and upon height above ground the 

 conditions will be varying constantly with variations in the 

 ground contours and the shape of the ground. 



Airmen have become accustomed to speak of so-called 

 pockets or holes in the air as offering unexpected obstacles to 

 aviation. We have at present no meteorological name for a 

 hole in the air and do not know the peculiarities of the air 

 currents in it ; we do know that gustiness, represented by 

 change of velocity above, must offer difficulties in the way of 

 alteration of vertical force and of longitudinal stability and that 

 gustiness, as measured by the range of velocity in a few seconds, 

 though very irregular, is roughly proportional to the average 

 velocity and depends upon exposure. When, therefore, in a 

 flight, a position of stronger and gustier wind is reached it is 

 inevitable that a greater strain should be placed upon the air- 

 manship of the pilot and the aeroplane may easily get beyond 

 his control. 



We do not as yet know the limits of gustiness which different 

 forms of aeroplanes can stand without becoming unmanageable; 

 we may, however, feel sure that there are limits of gustiness 

 which it is unsafe for an airman to attempt to navigate and we 

 want the regular comparison of the experience of the airman 

 and the records of gustiness obtained from anemometer records 

 near the ground and by kite records higher up. 



The first stage of this inquiry is to find out more exactly 

 what the gustiness really amounts to. I have spoken of it as 

 represented by variations of velocity only but we want to know 

 how far that is a true representation. An anemometer which 



