308 ON AEKIAL LOCOMOTIOl^. 



forward speed of 30 miles per hour, just double that of its passive de- 

 scent; the rate of fall under these conditions will be decreased most 

 remarkably, probably to less than one- fifteenth part, or 88 feet per min- 

 ute, or 1 mile per hour. 



The annexed line represents transversely the plane 2 feet wide and 10 



feet long, moving in the direction of the arrow ^ 2feet3»— ^ 



with a forward speed of 30 miles per hour, or I ''^ --~-------^ 



2,640 feet per minute, and descending at 88 feet^ 



per minute, the ratio being as 1 to 30. Now, the particles of air caught 

 by the forward edge of the plane must be carried down eight-tenths of 

 an inch before they leave it. This stratum, 10 feet wide and 2,640 feet 

 long, will weigh not less than 134 pounds ; therefore the weight has 

 continually to be moved downwards 88 feet per minute from a state of 

 absolute rest. If the plane, with this weight and an upward rise of 

 eight- tenths of an inch, be carried forward at a rate of 30 miles per 

 hour, it will be maintained at the same level without descending. 



The following illustration, though referring to the action of surfaces 

 in a denser fluid, are yet exactly analogous to the conditions set forth 

 in air: 



Take a stiff rod of wood and nail to its end at right angles a thin lath 

 or blade about 2 inches wide. Place the rod square across the thwarts 

 of a rowing-boat in motion, letting a foot or more of the blade hang 

 perpendicularly over the side into the water. The direct amount of 

 resistance of the current against the flat side of the blade may thus be 

 felt. Next slide the rod to and fro thwart-ship, keeping all square \ the 

 resistance will now be found to have increased enormously ; indeed, 

 the boat can be entirely stopped by such an appliance. Of course the 

 same experiment may be tried in a running stream. 



Another familiar example may be cited in the leeboards and sliding 

 keels used in vessels of shallow draught, which act precisely on the 

 same principle as the plane or wing-surface of a bird when moving in 

 air. These surfaces, though parallel to the line of the vessel's course, 

 enable her to carry a heavy press of sail without giving way under the 

 side pressure, or makmg lee- way, so great is their resistance against 

 the rapidly passing body of water, which can not be deflected sideways 

 at a high speed. 



The succeeding experiments will serve further to exemplify the action 

 of the same i)rinciple. Fix a thin blade, say 1 inch wide and 1 foot 

 long, with its plane exactly midway and at right angles to the end of 

 a spindle or rod. On thrusting this through a body of water, or im- 

 mersing it in a stream running in the direction of the axis of the spin- 

 dle, the resistance will be simply that caused by the water against the 

 mere superficies of the blade. Next put the spindle and blade in rapid 

 rotation. The retarding effect against direct motion will now be in- 

 creased near tenfold, and is equal to that due to the entire area of the 

 circle of revolution. By trying the effect of blades of various 'widths, 



