ON AKUIAL LOCOMOTION. 307 



According to Sineaton's table of atmospheric resistance, to produce a 

 force of 1 pound on a square foot, the wind must move against the phiue 

 (or, which is the same thing, the plane against the wind), at the rate of 

 22 feet per second, or 1,.')20 feet per minute, equal to 15 miles per hour. 

 The resistance of the air will now balance the weight on the descending 

 surface, and consequently it can not exceed that speed. Now 22 feet 

 per second is the velocity acquired at the end of a fall of 8 feet, — a 

 height from which a well-knit man or animal may leap down without 

 much risk or injury. Therefore, if a man with parachute weigh together 

 143 pounds, spreading the same number of square feet of surface con- 

 tained in a circle 14.^ feet in diameter, he will descend at perhaps an 

 unpleasant velocity, but with safety to life and limb. 



It is a remarkable fact how this pro[)ortion of wing-surface to weight 

 extends throughout a great variety of the flying portion of the animal 

 kingdom, even down to hornets, bees, and other insects. In some in- 

 stances however, as in the gallinaceous tribe, including pheasants, this 

 area is somewhat exceeded, but they are known to be very i)oor flyers. 

 Residing as they do chiefly on the ground, their wings are only required 

 for short distances or for rai.^ing them or easing their descent from their 

 roosting-places in forest trees, the shortness of their wings preventing 

 them from taking extended fliglit. The wing-surface of the common 

 swallow is rather more than in the ratio of 2 square feet per pound, but 

 having also great length of pinion, it is both swift and enduring in its 

 flight. When on a rapid course this bird is in the habit of furling its 

 wings into a narrow compass. The greater extent of surface is i)roba- 

 bly needful for the continual variations of speed and instant stoppages 

 requisite for obtaining its insect food. 



On the other hand, there are some birds, particularly of the duck 

 tribe, whose wing-surfsice but little exceeds half a. square foot, or 72 

 inches per pound ; yet they may be classed among the strongest and 

 swiftest flyers. A weight of 1 pound suspended from an area of this ex- 

 tent would acquire a velocity due to a fall of IG feet, a height sufficient 

 for the destruction or injury of most animals. But when the plane is 

 urged forward horizontally, in a manner analogous to the wings of a 

 bird during flight, the sustaining power is greatly influenced by the 

 form and arrangement of the surface. 



In the case of per[)endicu]ar descent, as a parachute, the sustaining 

 effect will be much the same, whatever the flgiire of the outline of the 

 superficies may be, and a circle perhaps aflbrds the best resistance of 

 any. Take for example a circle of 20 sipiare feet (as possessed by the 

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

 the rate of perpendicular descent to 1,320 feet per minute. But instead 

 of a circle CL inches in diameter, if the area is bounded by a jJarallelo- 

 gram 10 feet long by 2 feet broad, and whilst at perfect freedom to de- 

 scend per[)cndicularly, let a force be applied exactly in a horizontal 

 direction so as to carry it edgeways, with the long side foremost, at a 



