AERO -DYNAMICS signifies the science which treats of the motion of the air, or of the mechanical effects of air put in motion. There are not in which this sci ence bears directly on manufacturing opera tions or the arts generally ; but we may pre sent a few considerations to skew its influence on moving machinery.
As soon as we begin to move we feel, more or less, the resistance of the air. And, since any body in moving through a fluid, as air, not only displaces a greater number of parti cles of the fluid in equal times, in proportion as it moves faster, but causes each particle to react against the body more powerfully in pro portion as the latter, by moving faster, strikes it with greater force ; it follows that the resist ance of a fluid at rest against a body moving in it, or the resistance of the fluid in motion against a body at rest, varies with the square of the velocity of the body, or of the air ; that is, if the velocity be suddenly made ten times as great, the resistance is made ten times ten, or a hundred times as great. And this is sufficiently near the truth for practical purposes when the velocities are eight or nine hundred feet in a second.
The resistance is nearly in the same pro portion as the surface exposed, but a little greater than this proportion on the larger surface ; that is, if we take two bodies of the same figure and material (two iron spheres for example), the surface of the second being twice that of the first, the resistance to the larger sphere is a little more than twice that to the smaller, the velocities 'being the same in both.
The round ends and sharp ends of solids suffer less resistance than the flat ends of the same. Thus, the sharp end or vertex of a cone is less resisted than the flat end or base.
Two solids, similarly formed on the end to wards the air, are not equally resisted unless the hinder parts are also similar.
If we suppose both the wind and the body to bo in motion, the resistance is variously modified according to the direction of the motions of the two. If the wind and the body move in the same with the same velocity, there is no resistance, for no air is displaced by the body. If the wind move 50 feet per second, and the. body 100 feet, the pressure on the body is the same as if it were at. rest, with a contrary wind of 50 feet per second blowing on it. If the wind and the body move in contrary directions, with velocities of 100 feet, the resistance is that of a wind of 200 feet per second ; and so on.
The following table shows in pounds avoir dupois, the pressure which different winds will exert upon a square foot of surface exposed directly against them. The first column is a rough representation of the second.
A few illustrations of the resistance and pressure of the air will be met with under AIR-GUN, BELLOWS, BLOWING MACHINE, SAILS, WrxnmiLL,