The elementary parts of machinery are but few, it is in their multifarious combinations that the mechanical inquirer finds such an unbounded region for research, and the class of machines specified above is not the least among the achievements of the mechanic. Be that as it may, it is certain that the term press is common to a myriad of contrivances, which resemble each other only in their adaptation for the process whence they deiive their generic title. At present, however, we will abstain from commenting upon those presses that perform their office by a rolling contact, and rather direct attention to those whose operation is by a direct impact. In nearly all these, however dissimilar they may otherwise be, either a screw or a cam, and in some instances both is the agent employed for transmitting and regulating the power. The cam is an element ot machinery well worthy the attentive consideration of all interested in mechanics, and who, at the present day is not? It is a contrivance in which sliding contact is employed to communicate motion from one part of a machine to another, bat its chief peculiarity and advantage is the ability that it confers of timing its effective action, although it is intermittent, so that the cam may act in unison with the rest of the mechanism, and thus, when several mutually dependent processes are being carried on by the same machine, the whole are effected with automatic precision. The skill of the contriver is displayed in giving a suitable shape to the cam, which it is scarcely necessary to remark, is a revolving disc, part of whose periphery is more distant from the axis than other parts, the varying distances being united by a curved line. In its most simple form, the shape of the cam is that of an eccentric, when one complete double stroke of a sliding bar is made for each revolution of the axis, and the effect is similar to that ot a crank. When the bar or follower is to make a number of strokes during each rotation of the axis, the cam has. on its periphery, a series of projections or lilts, and these are united by a curve more or less abrupt, according as the stroke of the follower is to be given more or less suddenly. But if it is to be uniform and constant, an involute curve will et-fect it, and where the movements are more complicated it is necessary to make the curve of a more complex shape. A beautiful example of this arrangement is given in a recent Envelope-Folding Machine, where, as it is of advantage to give equal impetus to the machine, the law of falling bodies is followed in the extent of motion given by the cam to a lever, but if this were to operate until the end of the motion a considerable shock would be given to the machine ; it is therefore arranged that after maintaining this velocity a certain time it then gradually decreases in the same ratio as it before increased. The large class of machines employed by stationers are well worthy of remark, and the majority of them are intended for stamping and embossing processes,—many of these presses being exceedingly powerful. Butdiverging from these we will glance at the common fly press, an instrument that is used in so many occupations. It is a most useful machine, which, independently of the punch or dies, may be considered as a means of giving a hard, unerring, perpendicular blow. The precision of the blow is caused by the slide, by which the punch is guided and its force is imparted by the heavy revolving fly attached to the screw of the press. When steam power is used to work these it is variously employed, in the case of a mint, twelve presses for cutting out the blanks fjr coin were arranged in a circle around a heavy fly-wheel, which revolved horizontally, it had one projecting cam, which caught successively the twelve radial levers fixed in the screws of the presses, and the screws were forced back by springs. Sometimes a crank motion is used, or in lieu, an oscillatng air cylinder with its piston. The lever is often suspended by a contrivance well known as the toggle or knee joint, and frequently by another superior arrangement, as employed in Dicks Anti-Friction Press, in the most powerful form of which a central roller is made to revolve, and carries with it two eccentric wheels or cams, one on each side, and having their bearings on the faces ot sectors which are likewise made to partake ot their motion, and so give the necessary pressure. Presses with the toggle-joint are perfectly suited to cutting out works with punches and bolsters, provided the relative thickness of the work and tools is such as to bring to bear the strongest point of the mechanical ac tion at the moment that the greatest resistance occurs in the work; but as the fly- press with a screw is in all cases powerful alike, irrespective of such proportions, the screw-press is more generally useful. The presses used to punch boiler plates afford a surprising proof of the power developed by this tool, and in its simplest form a long lever, moved by the hand, gives the requisite intensity to the descent of the punch. The saw-gumming machine is a trifle more complex, but when more powerful presses are required, the lever becomes a massive beam which is lifted by a cam in a manner analogous to the tilt-hammer. This last-named tool may likewise be classed as a press, as may also another machine which is rapidly replacing it. This latter, namely, the steam hammer, is merely a piston moving in a covered cylinder where steam is admitted, the piston rod is attached to a heavy hammer so that when the piston is forcfd to the top of the cylinder, and the steam then withdrawn, the hammer tails violently on the mass of hot iron placed on the anvil directly beneath the cylinder. The press has been employed to rivet boilers, when the pressure is given by a knee-joint impelled by a cam. and the forging of spin dies and similar articles is another of its recent applications. In this latter instance the machine is quite portable, occupying a space of 3 feet by 4 feet, and contains five or six sets of anvils and swages. The anvils are arranged in a row in the frame at the usual height from the ground, and each swage is fixed to the lower end of a vertical bar, moving between proper guides, so as to be capable of rising and falling through a small space above its anvil. Its horizontal axis passes across the upper ends ot these swage-bars, and has an eccentric for each, so that the unilorm rotation of this axis causes every one of the swages to rise and fall periodically in order. The workman has merely to heat the bar in the fire and hold it under the vibrating swage turning it or otherwise changing its position according to the form he wishes to produce. It is stated that the machine will perform the labor of three men and their assistants or strikers, and will complete its work in a very superior manner and with great rapidity. Thus a piece of round iron 1 j inch in diameter was reduced to a square of f inch, 2 feet 5 inches long at one heat.