a 
wey —— ‘ge: 
of thirty feet per second. ie 
1s thirty feet per second, will be 12 sin? @ lbs., and because its. . 
Weight remains the same the plane will cease to be in equilibrium. 
- An increase in the velocity of the descent of the plane in the — 
Mto of Vsin? 6: t would restore the equilibrium and the plane — 
1886.] The Mechanics of Soaring. 533 
surface is indicated, by the weight of the body at that point. 
Weight, therefore, or gravity, is a constant force, and difters 
in no respect from any other constant force. 
It is found by experiment that a current of air, having a 
velocity of thirty feet per second, and meeting a stationary 
plane, the projection of which, in the direction of the motion, 
has a superficial area of one square foot, exerts upon the plane, 
in the direction of the current, a force = 2 lbs. X sin? 0, where 
0 denotes the angle which the plane makes with the direction of 
the motion; and for different planes the force is approximately 
Proportional to the areas of the planes, so that on a plane _ 
one foot wide and six feet long, as supposed by Mr. Lan- 
caster, when the current is normal to the plane, the force ex- 
erted upon the plane by a velocity of thirty feet per second will 
be twelve pounds, This is the value of the special force assumed 
by Mr. Lancaster, and whether we consider the twelve pounds 
Pressure as resulting from a constant atmospheric current or 
any other cause is obviously indifferent. 
If then we suppose the weight of the plane to be twelve 
pounds, and its descent vertical through a quiescent atmos- 
Phere, while the plane surface is horizontal, the case will be that 
of a falling body in a resisting medium, and when the plane shall 
have acquired a velocity of thirty feet per second, we know, from 
the €xperiments above referred to, that the plane will meet a con- 
stant resistance of twelve pounds, and this being the weight of 
the plane it will thenceforth descend with the uniform velocity 
If while thus descending, in equilibrium, a momentary hori- ` 
zontal force, the intensity of which is equal to gravity, be im- 
Pressed upon the plane, because it would impart to one pound a 
velocity of thirty-two feet per second, it will therefore impart 
to the plane, the weight of which is twelve pounds, a velocity _ 
of only tz X 32 ft, or 234 ft. per second, instead of 1000 ft. per 
Second as assumed by Mr. Lancaster. s 
If now the plane be tilted so as to make an angle @ with the 
vertical, the vertical pressure it will encounter, when the velocity _ 
RT 
