376 Flying Machines. 



limb. Swallows have a wing surface of two square feet to the 

 pound; some of the duck tribe, which fly well, little more 

 than half a square foot, or 72 inches to the pound. If such 

 birds allowed themselves to fall perpendicularly, with out- 

 stretched wings, they would reach the ground with an injurious 

 velocity, but by descending obliquely, they alight with ease and" 

 safety. This combination of a horizontal motion with a per- 

 pendicular one is of the greatest importance ; and Mr. Wenham 

 observes, " In the case of perpendicular descent, as a parachute, 

 the sustaining effect will be much the same, whatever the 

 figure of the outline of the superficies may be, and a circle 

 affords, perhaps, the best resistance of any. Take, for example, 

 a circle of twenty square feet (as possessed by the pelican), 

 loaded with as many pounds. This, as just stated, will limit the 

 rate of perpendicular descent to 1320 feet per minute. But 

 instead of a circle sixty-one inches in diameter, if the area is 

 bounded by a parallelogram ten feet long by two broad, and 

 whilst at perfect freedom to descend perpendicularly, let a 

 force be applied exactly in a horizontal direction, so as to carry 

 it edgeways, with the long side foremost, at a forward speed of 

 thirty miles an hour — just double that of its passive descent — 

 the rate of fall, under these conditions, will be decreased most 

 remarkably, probably to less than one-fifteenth part, or eighty- 

 eight feet per minute, or one mile per hour.'''' This diminution 

 of the descending velocity is occasioned by the resistance of 

 the mass of air moved by the parachute in its horizontal course, 

 and which necessarily becomes greater in proportion to the 

 width of the parachute. 



Among the experimental illustrations suggested by Mr. 

 Wenham is the action of a thin blade, one inch wide and a foot 

 long, fixed at right angles to a spindle on which it can be 

 turned. If such an apparatus is immersed in a stream running 

 in the direction of the spindle, and held at rest, the force 

 which the blade has to resist will be simply that of the water 

 current acting on its surface, and the current will be checked 

 to a corresponding extent. If, however, the spindle and blade 

 are made to rotate rapidly, " the retarding effect against direct 

 motion will now be increased over 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, it will be found that, for the 

 purpose of effecting the maximum amount of resistance, the 

 more rapidly the spindle revolves the narrower may be the 

 blade." 



It will be evident, that if a column of air were rotating in 

 the same direction and with the same velocity as that of the 

 vane and spindle, the movement of the vane would not be 

 resisted by the air, and just to the extent to which the revolving 



