ON MODES OF CHANGING THE FORMS OF BODIES.

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simple pressure; the degree of its efficacy depends, however, ou the degree ofcompressibility of the substance. Thus, if a heavy body fall from a certainheight, so as to acquire a momentum, in consequence of the force of gravity,it will ultimately exert on the substance upon which it falls, a force about asMuch greater than its weight, as the space, through which the .surface of thesubstance struck is depressed, by means of the impulse, is less than twice theheight from which the body has fallen; and unless either the substance isVe >'y compressible, or the height very small, this force must be incomparablySweater than the pressure of the weight only.

For a printing press, a single heavy roller is sometimes made to pass overIhe paper, when it has been laid on the types; and since the whole action ofSll ch a roller is confined to a small part, at any one time, it is said to exertsufficient force, and to perform its work more equably than a common press,^ u t its operation must be comparatively slow. A common mangle foi linena cts nearly in a similar manner. In calendering mills, the force ol a spiing is^’Mployed, for exerting a pressure on the block, with which the mateiials ateglazed.

Fhe copper plate printing press, and the machine for copying letters, are com-l )0 sed of two rollers, parallel to each other, pressing on the substance which,s Mterposed, and which is brought into its situation partly by the friction of

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surface of the roher, and partly by external force.

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tle rollers, by which sugar canes are pressed, are in general situated verti-the middle one of three being turned by horses, by mules, or by water,the canes being made to return round it, so as to pass through both in-1 slices in succession. It appears to be of some advantage in presses of thisthat all the rollers should be turned independently of their action on theAerials interposed, since the friction of two rollers may tend to draw the^teiials into the space between them, with more regularity and greater force,a u the action of a single roller would do. For this reason, it may be ad-e to retain the toothed wheels turning the rollers, even when their axes

FiVo 0t flrm, y Fixed, hut held together by an elastic hoop. (Plate XVIII.£■ 231 .)