square of the velocity, the least resistance that such a ball would meet with, when moving with a velocity of 1600 feet would be equal to the pressure of 4171b, and that independent of the pressure of the atmosphere itself on the fore part of the ball which would be 487lb more, as there would be no pres-. sure from the atmosphere on the hinder part, in the case of so great a velocity as 1600 feet per second. So that the whole resistance would be more than 900lb to such a velocity. 9. Having said, in the last article, that the pressure of the atmosphere is taken entirely off the hinder part of the ball moving with a velocity of 1600 feet per second; which must happen when the ball moves faster than the particles of air can follow by rushing into the place quitted and left void by the ball, or when the ball moves faster than the air rushes into a vacuum from the pressure of the incumbent air; let us therefore inquire what this velocity is. Now the velocity with which any fluid issues, depends on its altitude above the orifice, and is indeed equal to the velocity acquired by a heavy body in falling freely through that altitude. But, supposing the height of the barometer to be 30 inches, or 21 feet, the height of a uniform atmosphere, all of the same density as at the earth's surface, would be 21 × 13.6 × 8334 or 28333 feet; therefore/16:28333:: 32: 8/28333=1346 feet, which is the velocity sought. And therefore, with a velocity of 1600 feet per second, or any velocity above 1346 feet, the ball must continually leave a vacuum behind it, and so must sustain the whole pressure of the atmosphere on its fore part, as well as the resistance arising from the vis inertia of the particles of air struck by the ball. 10. On the whole, we find that the resistance of the air, as determined by the experiments, differs very widely, both in respect to its quantity on all figures, and in respect to the proportions of it on oblique surfaces, from the same as determined by the preceding theory: which is the same as that of Sir Isaac Newton, and most modern philosophers. Neither should we succeed better if we have recourse to the theory given by Professor Gravesande, or others, as similar differences and inconsistencies still occur. We conclude therefore, that all the theories of the resistance of the air hitherto given, are very erroneous. And the preceding one is only laid down, till further experiments, on this important subject, shall enable us to deduce from them another, that shall be more consonant to the true phænomena of nature. ON ON THE MOTION OF MACHINES, AND THEIR be ART. 1. When forces acting in contrary directions, or in any such directions as produce contrary effects, are applied to machines, there is, with respect to every simple machine (and of consequence with respect to every combination of simple machines) a certain relation between the powers and the distances at which they act, which, if subsisting in any such machine when at rest, will always keep it in a state of rest, or of statical equilibrium; and for this reason, cause the efforts of these powers when thus related, with regard to magnitude and distance, being equal and opposite, annihilate each other, and have no tendency to change the state of the system to which they are applied. So also, if the same machine have been put into a state of uniform motion, whether rectilinear or rotatory, by the action of any power distinct from those we are now considering, and these two powers be made to act upon the machine in such motion in a similar manner to that in which they acted upon it when at rest, their simultaneous action will preserve it in that state of uniform motion, or of dynamical equilibrium: and this for the same reason as before, because their contrary effects destroy each other, and have therefore no tendency to change the state of the machine. But, if at the time a machine is in a state of balanced rest, any one of the opposite forces be increased while it continues to act at the same distance, this excess of force will disturb the statical equilibrium, and produce motion in the machine; and if the same excess of force continues to act in the same manner, it will, like every constant force, produce an accelerated motion; or if it should undergo particular modifications when the machine is in different positions, it may occasion such variations in the motion as will render it alternately accelerated and retarded. Or the different species of resistance to which a moving machine is subjected, as the rigidity of ropes, friction, resistance of the air, &c. may so modify a motion, as to change a regular or irregular variable motion into one which is uniform. 2. Hence then the motion of machines may be considered as of three kinds. 1. That which is gradually accelerated, which obtains commonly in the first instants of the communication. 2. That which is entirely uniform. 3. That which is alternately accelerated and retarded. Pendulum clocks, and machines which are moved by a balance, are related to the 1 the third class. Most other machines, a short time after their Defs. 1. When in a machine there is a system of forces or of powers inutually in opposition, those which produce or tend to produce a certain effect are called, movers or moving powers; and those which produce or tend to produce an effect which opposes those of the moving powers, are called resistances. If various movers act at the same time, their equivalent (found by means of prob. 7, Motion and Forces) is called individually the moving force; and, in like manner, the resultant of all the resistances reduced to some one point, the resistance. This reduction in all cases simplifies the investiga tion. 2. The impelled point of a machine is that to which the action of the moving power may be considered as immediately applied; and the working point is that where the resistance arising from the work to be performed immediately acts, or to which it ought all to be reduced. Thus, in the wheel and axle, (Mechan. prop. 32), where the moving power p is to overcome the weight or resistance w, by the application of the cords to the wheel and to the axle, в is the impelled point, and a the working point. A VOL. II. 54 3. The : 3. The velocity of the moving power is the same as the velocity of the impelled point; the velocity of the resistance the same as that of the working point.. 4. The performance or effect of a machine, or the work done, is measured by the product of the resistance into the velocity of working point; the by the product of the moving mentum of impulse is measured impelled point. force into velocity of the These definitions being established we may now exhibit a few of the most useful problems, giving as much variety in their solutions as may render one or other of the methods of easy application to any other cases which may occur. PROPOSITION I, F If R, and r be the distances of the power P, and the weight or resistance w, from the fulcrum r of a straight lever: then will the velocity of the power and of the weight at the end of any Rrp r2 W R2Praw time t he R2P-Rrw gt, and R2 p + r2 w and inertia of the lever itself not being considered. If the effort of the power ba gt, respectively, the weight ly, the difference between this value of P, and its actual value, or P R w, will be the force which tends to move the lever. And because this power applied to the point a accelerates the masses p and w, the mass to be substituted for w, in the point. A, must be w, (Mechan. prop. 50) in order that this mass R2 at the distance R may be equally accelerated with the mass R w at the distance R. Hence the power P w will accele rate the quantity of tter: force F ==(P——w)÷(p+2w Ꭱ . t w; and the accelerating because veloc. of r: veloc. of w: Rr, therefore veloc. of Corol. 1. The space described by the power in the time t, Cor. 2. If R -r :: n : 1, then will the force which acce Cor. 3. If at the same time the inertia of the moving force P be 0, as in muscular action, the force accelerating Cor. 4. If the mass moved have no weight, but possesses inertia only, as when a body is moved along a horizontal plane, the force which accelerates a will be = A rn2 Pn2 +w And either of these values may be readily introduced into the investigation. Cor. 5. The work done in the time t, if we retain the ori ་་ •g*t. Cor. 6. When the work done is to be a maximum, and we wish to know the weight when P is given, we must make the fluxion of the last expression =0. Then we shall have, rR3p2 — 2r2 R2 PW — r1 w2=0 and w=P XI✓ ( + ~ ) — — ] · 20 R4 R3 R2 22 Cor. 7. If R : T n: 1, the preceding expression will become wP × [√(n1+n3)—n2]·· Cor. 8. When the arms of the lever are equal in length, that is, when n = 1, then is w = P X (√√⁄2 −1) = ·414214p, or nearly of the moving force. 5 Scholium. If we in like manner investigate the formulæ relating to motion on the axis in peritrochio, it will be seen that the expressions correspond exactly. Hence it follows, that when it is required to proportion the power and weight so as to obtain |