APPLIED MECHANICS.] MECHANICS centre of percussion is at the point where the perpendicular OG meets 0. 136. Impact. Impact or collision is a pressure of short duration exerted between two bodies. For the detailed investigation of its laws the reader is referred to MECHANICS, p. 719, 180 sq. The effects of impact are sometimes an alteration of the distribu tion of actual energy between the two bodies, and always a loss of a portion of that energy, depending on the imperfection of the elasticity of the bodies, in permanently altering their figures, and producing heat. The determination of the distribution of the actual energy after collision and of the loss of energy is effected by means of the following principles : I. The motion of the common centre of gravity of the two bodies is unchanged by the collision. II. The loss of energy consists of a certain proportion of that part of the actual energy of the bodies which is due to their motion relatively to their common centre of gravity. Unless there is some special reason for using impact in machines, it ought to be avoided, on account not only of the waste of energy which it causes, but from the damage which it occasions to the frame and mechanism. CHAPTER III. PURPOSES AND EFFECTS OF MACHINES. 137. Observing Machines and Working Machines. The present chapter must necessarily be limited to some very general observa tions on the principal classes into which machines may be divided, with reference to their purposes and effects, leaving details of par ticular examples for treatment under the respective special headings. Machines may be divided, in the first instance, into two great divisions, viz. : I. Observing Machines, in which either the modification of motion alone, or the balancing of forces alone, is the object in view, the performance of work being either null or incidental, and being limited to that which arises from the resistance of the machine. II. Working machines, in which the performance of work is the main object. 138. Classification of Observing Machines. Observing machines might very properly have been classed as instruments, being de signed to aid the human senses and memory in obtaining and recording information. They may be divided, in the first instance, into four classes, according as the subject of observation by their aid is number, measure, or weight, into A. Counting machines. B. Measuring machines. C. Copying and drawing machines. D. Weighing machines. And to these may be added a fifth class, in which the functions of the first four are more or less combined, viz.: E. Recording machines. 139. (A. ) Counting Machines. The most important as well as the most common of counting machines are time-keepers, which count and indicate the numbers of oscillations of bodies which oscillate isochronously (viz., pendulums forelocks, balance-wheels for watches and marine chronometer*) so as to measure time. In constructing such machines, the objects to be aimed at are the exact isochronism uf the pendulum or balance, and the equable action of the motive power, so that it shall overcome the friction of the mechanism without affecting the rate. Other counting machines count the oscillations of the beam of a steam-engine, or the revolutions of the cylinder of a gas-meter or of the wheel of a water-meter. Others perform additions, subtractions, and multiplications, and of these the most elaborate kind (as, e.g., that of Babbage) compute tables of functions by the addition of differences. 140. (B.) Measuring Machines. Measuring machines are pieces of mechanism, by means of which the motion of some body of the nature of an index through some geometrical magnitude, such as a distance or an angle, is connected with some other motion, either equal or greater or smaller in some given ratio, and capable of being more readily compared with some standard of measure. To this class belong all those astronomical and surveying instru ments in which the motion of a line of sight (generally the line of collimation of a telescope) through a given angle is connected with the motion of an index or vernier round a corresponding arc of a graduated circle ; also those micrometers in which the advance of the end of a screw of fine pitch is measured by observing the simul taneous arc of rotation of a graduated circle attached to it. Such micrometers have attained increased importance by the discovery of Whitworth, that the mechanical magnifying of small distances by a train of screws affords a more accurate means of measurement than optical magnifying by the microscope, and by the perfection to which that engineer has brought that art of accu rate workmanship which is necessary in order to render mechanical magnifying possible. Amongst measuring machines are included the plalomctcrs or planimctcrs of Sang, Morin, and Clerk Maxwell, which measure areas by means of mechanism. The amount of resistance in a measuring machine should be perfectly uniform, and sufficiently great to prevent accidental forces from disturbing the machine, without being so great as to render it inconveniently still . To combine these objects requires great accuracy of workmanship, together with strength and rigidity in the structure of the frame and mechanism. 141. (C.) Copying and Draw i /i g Machines. In copying machines for enlarging or reducing drawings there is usually a combination of levers and linkwork connecting a tracing-point, which is moved over the lines of the original figure, with a drawing-point, which draws the copy in such a manner that the velocity ratio of their motions is a given constant quantity, and that the directions of their motions make a constant angle. Mechanism depending for its principle on the theory of the composition of rotations is used to draw ellipses, epicycloids, epitrochoids, and other curves. 142. (D.) Weighing Machines. In weighing machines the motion of the mechanism is used only for the purpose that its cessation, or its becoming an oscillation about a certain position, may indicate the equilibrium of the forces applied to the machine. Those forces may either be weights, which are to be compared with each other, or forces of other kinds, to be compared directly or indirectly with weights. The machine for comparing weights which is capable of the most minute accuracy is also the simplest, being the balance, in which the equality of two weights is ascertained by their balancing each other at the ends of a lever of equal arms. In tht steelyard, consisting either of one lever or of a train of levers, the unknown weight has an unchangeable point of application, and is compared with a known weight by shifting the latter along the lever to hieh it is applied until the machine is balanced ; the ratio of the weights is then the reciprocal of the velocity ratio of their points of applica tion. The steelyard is more convenient for weighing very heavy loads than the balance, but is not capable of such minute accuracy. It is essential to accuracy in balances and steelyards that the friction should be less than the smallest admissible amount of error. To diminish : the friction as much as possible, the axes of motion are all knife-edges, as they are termed, of steel or hardened ircn, resting on hard surfaces of hardened iron or steel for ordinary purposes, and of some hard mineral, such as agate, for scientific purposes. The weight of a- column of fluid is determined by balancing it against a column of fluid whose weight is known, as in the barometer, where the weight of a column of the atmosphere is balanced against that of a column of mercury. Weights are compared with each other indirectly, and other forces compared with weights, by means of their effects in bending a spring, a convenient method, but not susceptible of minute accuracy. The elastic pressure exerted by a fluid may be compared with weight, either by balancing the pressure against the weight of a column of a liquid, or by maintaining a piston in equilibrium against that pressure, by means of a weight pressing it directly, or of a weight acting through a steelyard, or of the elasticity of a spring which has been compared with weights. 143. (E.) Recording Machines. Recording machines may be divided into two classes -.self -registering instruments, which, by the aid of clockwork, record measurements either of space or of force, together with the instants of time at which these measurements were made ; and dynamometers, already mentioned in Chap. II. of this article, which register measurements of force, together with the space through which it has acted, thus recording energy or work. 144. Working Machines Classed. The object or purpose of work ing machines is to perform useful work ; and their classification relatively to their objects and purposes is founded on the kind of useful work which they perform. In this point of view they may be classed as follows : A. Machines for lifting or lowering solid weights. B. Machines for the horizontal transport of weights, either com bined or not with lifting or lowering. C. Machines for projecting solids. D. Machines for lifting fluids. E. Machines for propelling or projecting fluids. F. Machines for dividing bodies. G. Machines for shaping bodies by removing portions of them. H. Machines for shaping bodies by pressure. I. Machines for uniting bodies into fabrics. J. Machines for printing. K. Machines for producing sound. L. Miscellaneous machines. It is not pretended that the above classification (taken to a con siderable extent from the writings of Young and of Bubbage) exhausts all kinds of machines ; it is brought forward merely as an attempt to introduce method to a certain extent into a subject which would otherwise be exceedingly confused. 145. (A.) Machines for Lifting and Lowering Solids. lue m.
common machines of this class are caftans, cranes, aud windlasses.