THE I\Ti:i:\'\l. WORK OF THE \VI\I>. 17 



tion), wlici jone of the velocities is negligibly small, and where successive pulsations 



in the same directions are separated by intervals of calm. 



A frequent alternation of velocities, united with constancy of absolute direc- 

 tion, lias previously been shown here to be the ordinary condition of the wind's 

 motion ; but attention is now particularly called to the fact that while these un- 

 equal velocities may be in the same direction as regards the surface of the earth, yet 

 as regards the mean motion of the wind they are in opposite directions, and will 

 produce on a plane, whose inertia enables it to sustain a sensibly uniform motion 

 with the mean velocity of this variable wind, the same lifting effect as if these same 

 alternating winds were in absolutely opposed directions, provided that the (constant) 

 inclination of the plane alternates in its aspect to correspond with the chauges in 

 the wind. 



It may aid in clearness of conception, if we imagine a set of fixed co-ordinates X 

 Y Z passing through 0, and a set of movable co-ordinates x y z, moving with the 

 velocity and in the direction of the mean wind. If the moving body is referred to these 

 first only, it is evidently subject to pulsations which take place in the same directions 

 on the axis of X, but it must be also evident that if referred to the second or mova- 

 ble co-ordinates, these same pulsations may be, and are, in opposite directions. This, 

 then, is the case we have just considered, and if we suppose the plane to change 

 the aspect * of its (constant) inclination as the direction of the pulsations changes, 

 it is evident that there must be a gain in altitude with every pulsation, while the 

 plane advances horizontally with the velocity of the mean wind. 



During the period of maximum wind velocity, when the wind is moving faster 

 than the plane, the rear edge of the latter must be elevated. During the period of 

 minimum velocity, when the plane, owing to its inertia is moving faster than the 

 wind, the front edge of the plane must be elevated. Thus the vertical component 

 of the wind pressure, as it strikes the oblique plane, tends, in both cases, to give it 

 a vertical upward thrust. So long as this thrust is in excess of the weight to be 

 lifted, the plane will rise. The rate of rise will be the greatest at the beginning of 

 each period, when the relative velocity is greatest, and will diminish as the resist- 

 ance produces "drift" ; i. e., diminishes relative velocity. The curved line O B in 

 the vignette represents a typical path of the plane under these conditions. 



It follows from the diagram (Fig. 1.) that, other things being equal, the more 

 frequent the wind's pulsations, the greater will be the rise of the plane; for since, 

 during each period of steady wind, the rate of rise diminishes, the more rapid the 



* We do not for the moment consider how this change of aspect is to be mechanically effected ; 

 we only at present call attention to the fact that it involves, in theory, no expenditure of energy. 



