ON MODES OF CHANGING THE FORMS OF BODIES. 221 



simple pressure; the degree of its efficacy depends, however, on the degree of 

 compressibility of the substance. Thus, if a heavy body fall from a certain 

 height, 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 as 

 much greater than its weight, as the space, through which the surface of the 

 substance struck is depressed, by means of the impulse, is less than twice the 

 height from which the body has fallen; and unless either the substance is 

 very compressible, or the height very small, this force must be incomparably 

 greater than the pressure of the weight only. 



For a printing press, a single heavy toller is sometimes made to pass over, 

 the paper, when it has been laid on the types; and since the whole action of;.' 

 such a roller is confined to a small part, at any one time, it is said to .exert 

 sufficient force, and to perform its work more equably than a common press; 

 but its operation must be comparatively slow. A common mangle for linen 

 acts nearly in a similar manner. In calendering mills, the force of a spring is 

 employed, for exerting a pressure on the block, with w.hich the materials arc 

 glazed. 



The copper plate printing press, and the m-achi«e for copying letters, are com- 

 posed of two rollers, parallel to each other, pressing on the substance which 

 is interposed, and which is brought into its situation partly by the friction of 

 the surface of the roLer, and partly by external force. 



The rollers, by which sugav canes are- pressed, are in general situated verti- 

 cally, the middle one of three being turned by horses, by mules, or by water, 

 and the canes being made to return round it, so as to pass through both in- 

 terstices in succession. It appears to be of some advantage in presses of this 

 kind, that all the rollers should be turned independently of their action on the 

 materials interposed, since the friction of two rollers may tend to draw the 

 materials into the space between them, with more regularity and greater force, 

 than the action of a single roller would do. For this reason, it may be ad- 

 visable to retain the toothed wheels turning the rollers, even when their axes > 

 are not firmly fixed, but held together by an elastic hoop. (Plate XVIII.., 

 Bg. 231.) 



