458 THE EMPIRE OF THE AIR. 



at tlie expense of height. This last procedure is the one generally 

 employed by the birds, but as we know that they can do better upon 

 occasion, it might be well to experiment. Birds are like all inferior 

 creatures: they do not like to tax their brain witli sustained attention, 

 and the circling- sweep brings no tax upon their heads, while it enables 

 them to search for food. Inasmuch as man will only desire to get 

 forward over the ground, and possesses greater faculties for combina- 

 tions than birds, the care in balancing required by direct ascension 

 will be mere sport to him. 



With the wind abeam nothing is more simple. All there is to do, so 

 to speak, is to allow ourselves to sail. The sailing l)irds while doiug 

 this wear a happy look; the observer feels that they are laboring neither 

 with body nor with brain, especially if the wind be brisk enough to 

 sustain them well. If the breeze is feeble then they have to take to 

 circling from time to time; but when it is sufficiently strong sailing on 

 a quarter wind is certainly the most convenient, and it is the first mode 

 which will be successfully employed by man. It will be the system 

 causing the least difficulty, and which man will utilize much more than 

 the bird, the former being always anxious to get to his journey's end. 



A brisk wind ought to permit of direct ascent, even if blowing from 

 the direction sought; by fiiciug the wind and rising while drifting back, 

 or even by receiving it in the rear of the a'roplane, that is to say by 

 gliding with the wind during a lull, and turning an angle and descend- 

 ing slightly during a puff of vrind. These two different mananivers are 

 performed by the sailing birds, but they employ them so rarely that 

 they nmy be said not to be in their line. With a good Avind, when we 

 desire to ])roceed in its direction, both the ascent and the horizontal 

 progress will be achieved in the latter manner. 



Summing up, even admitting that man shall invent no new manreu- 

 vers, he will nevertheless have a choice among many, and their combi- 

 nation will constitute what ought to be termed the human type of 

 flight. We may condense our studies into a smaller compass and say: 

 When the aiH-oplane enters into motion, its center of pressure varies in 

 the direction of that motion and is displaced by an amount which 

 varies with the speed. 



With machines ''heavier than the air" aerial navigation may be com- 

 passed with two separate classes of apparatus: (1 ) By machines with 

 propellers. (2) I)y aeroi^lanes without proi)ellers. 



The first class is (juite outside of my present design. Mechanical 

 science will eventually furnish quite a number of different solutions; 

 such as fiapping wings, propelling screws, rocket propulsion, etc. 



The second class — that is to say, the aeroplane without a propeller — 

 it is the object of the i)resent essay to promote. What luis been stated 

 herein permits me to affirm that in the flight of the sailing birds (the 

 vultures, the eagles, and other birds which Hy without beating the 

 air), ascension is produced by the skillful use of the force of the wind, 



