312 ON AERIAL LOCOMOTION 



to a small spring steelyard, wbicli served iis a handle to pull it out by. 

 The weight or degree at which the index had been drawn was after- 

 wards ascertained by the mark left thereon by a pointed brass wire. It 

 is not necessary to know the time occupied in drawing out the string, 

 as this item in the estimate may be taken as the duration of the ascent; 

 for it is evident that if the same force is re-applied at the descent, it 

 would rise again, and a repeated series of these impulses will represent 

 the power required to prolong the flight of the instrument. It is there- 

 fore requisite to know the len gth of string and the force applied in 

 pulling it out. The following are the data : 



Diameter of screw inches.. 8+ 



Weight of screw grains . . 3yf) 



Length of string drawn out feet.. 2 



Force employed pounds.. 8 



Duration of flight seconds.. IG 



From thisit may be computed that, in order to maintain the flight of the 

 instrument, a constant force is required of near GO foot-power per minute 

 — in the ratio of about 3 horse-power for each hundred pounds raised 

 by such means. The force is perhaps over-estimated for a larger screw, 

 for as the size and weight is increased the power required would be 

 less than in this ratio. The result would be more satisfactory if tried 

 with a sheet iron screw impelled by a descending weight. 



Methods analogous to this have been proposed for attempting aerial 

 locomotion •, but experiment has shown that a screw rotating in the air 

 is an imperfect principle for obtaining the means of flight and support- 

 ing the needful weight, for the power required is enormous. Sui)pose 

 a machine to be constructed having some adequatesupplyof force, the 

 screw rotating vertically at a certain velocity will raise the whole. 

 When the desired altitude is obtained, nearly the same velocitj'^ of rev- 

 olution and the same excessive power must be contioued, and con- 

 sumed entirely in "slip," or in drawing down a rapid current of air. 



If the axis of the screw is slightly inclined from the perpendicular, the 

 whole machine will travel forward. The "slip," and consequently the 

 power, is somewhat reduced under these conditions; but a swift forward 

 s )eed can not be effected by such means, for the resistance of theinclined 

 disk of the screw will be very great, far exceeding any form assimilat- 

 ing to the edge of the wing of a bird. But, arguing on the supposition 

 that a forward speed of 30 miles an hour might thus be obtained, even 

 then nearly all the power would be expended in giving an unnecessary 

 and rapid revolution to an immense screw, capable of raising a weight, 

 say, of 200 pounds. The weight alone of such a machine must cause it 

 to frtil, and every revolution of the screw is a subtraction from the much 

 desired direct forward speed. A simple narrow blade or inclined plane 

 propelled in a direct course at this speed (which is amply sufficient for 

 sustaining heavy weights) is the best — and in fact the only means of 

 giving the maximum amount of supporting power with the least possi- 



