364 
POPULAR SCIENCE REVIEW. 
have to decide what that area must he in proportion to the 
weight to be sustained. 
It may be assumed that the limit can be determined by the 
condition, that supposing the muscular force of the man or bird 
should fail, that the wing surface or plane will afford sufficient 
resistance in a perpendicular descent, while the animal is passive, 
to prevent him from being injured on reaching the earth. If 
-one square foot of extension be taken for each pound weight, 
the resistance of the air will limit the rate of descent to twenty- 
two feet in one second of time ; this will be sufficient for safety, 
as it is the velocity acquired by a body at the end of a fall of 
eight feet — a height from which an active man may leap down 
with impunity. It has been ascertained that this area of one 
foot for each pound is the average wing surface of most flying 
animals. It must, however, be borne in mind that large wing 
surface does not indicate great facility of flight, as some of the 
swiftest and most enduring flyers have only half that area ; let 
the argument, therefore, be based upon the proportion of one 
pound per square foot of surface. If this is arranged in the 
form of a parachute, and the total weight of the man and 
apparatus be taken at 200 lbs., we then have this weight 
overcoming the resistance of the atmosphere at a falling speed 
of twenty-two feet per second, or 1,320 feet per minute. This 
gives us a definite measure of units of force, generally estimated 
at what is termed “ foot-pounds,” that is the total force, weight, 
or resistance in pounds, multiplied by the rate or velocity in feet 
per minute. By so multiplying these given quantities we have 
the enormous force exerted of 264,000 foot-pounds, and dividing 
this by the Watt standard of horse-power, viz. 33,000 lbs. 
raised one foot high per minute, there results eight horse-power, 
or about the strength of forty men. Now the whole of this 
force is represented by the action and reaction of the opposing 
bodies — the resistance of the plane to the air, or conversely the 
air to the plane, for precisely the same amount of power would 
be consumed if the weighted plane were itself stationary and 
the air rushing against it at the rate of twenty-two feet per 
second. This eight horse-power, therefore, represents the force 
expended, and that would be required to keep the body weigh- 
ing 200 lbs. sustained at a uniform height in air, on an area of 
the same number of square feet. 
If the estimate is taken with another area, the result will 
differ. With half a square foot per pound the descent will be 
about 1,800 feet per minute, and the power expended nearly 
equal to that of eleven horses. Let us advance into the other 
extreme, and allow for the 200 lbs. a surface of 43,560 square feet 
— equal to one acre in extent. The descent would now be very 
slow — about one mile per hour, or eighty-eight feet per minute, 
