412 THE EMPIRE OF THE AIR. 



fore examine the ]>rol)lem lurtlier, and seek a Jiiuie satisfaetory solutiou 

 of tlie eireling problem. This we shall tind in studying' the effects 

 ]>roduced by irregular gusts of wind. 



The wind gust is the very essence of the uprise ; it is the magic w^aud 

 which, striking the child's hoop, keeps it upright in rolling, drives it 

 along, or raises it up to overleap elevations on its way. Supj)ose the 

 toy to l)e placed on a steep inclined descent ; gravity wUl cause it to 

 roll to the bottom. If beyond this an ascending plane follow^s, the lioop, 

 urged forward by momentum of acquired velocity, will rise to a height 

 equal to that of fall, minus the losses by friction on the soil and by air 

 resistance. But if, instead of utilizing gravity alone, we accelerate the 

 hoop with the wand, it will run up much higher than the point it 

 started from. 



Let us suppose further, when the hoop is about to ascend, we can 

 displace the ascending plane, in contrary direction to the toy's course, 

 so that the plane shall glide under the hoop, then we would still more 

 assist the ascension, by adding a supplementary force, independent of 

 the others, and whose resultant would likewise be an uprise. 



Let us nc^w re-consider the action of the vivifying current of air u]>on 

 the bird. 



If the gust of wind occurs just where the bird is going with the wind, 

 its effect is akin to the blow of the wand on the hoop; it stores up 

 energy and economizes descent, hence the bird profits to that extent. 



If the gust occurs when the bird faces the wind, then the sustaining- 

 plane is gliding beneath him, and the resulting i^ressure causes him to 

 ascend; therefore again profit results in an uprise, nowise connected 

 with antecedent fall. 



If the wind gust occurs when the bird is on the quarter sweep, 

 forward or back of the wind's course, it still exerts contributory rise; 

 there is always, in each case, an impulse, a thrust from foreign source, 

 which the l)ird profits by; or else a saving of acquired momentum, 

 which the creature transforms into uprise. 



But, after all, these explanations avail only for the iiublic, curious to 

 know why. They neither corroborate nor disprove the facts. Whether 

 we understand and mathematically demonstrate the mechanics of sailing 

 flight, or whether we fail in the attemjit, the result is the same. There 

 remains always the demonstration produced by the Gieat Master, who 

 in His wisdom has implied: ''If you understand it, it is well; if you do 

 not understand, 'tis to be regretted; but in any case, look! that is the 

 way 'tis done ! I exhibit it all day long, not in a dark corner, but in 

 the blue; and if you can not eventually profit by the lesson, you will 

 really deserve never to join me in the skies." 



Thus acts the bird ; he demonstrates. We see the demonstration ; and 

 what more can we do with a formula which leads to no result? What 

 testimony is an explanation more or less clear? Can there remain a 

 doubt as to the fact of sailing flight, when the proof is evident and visible 



