169 



LECTURE XIX. 



ON MODES OF CHANGING THE FORMS OF BODIES. 



THE corpuscular forces by which bodies retain their peculiar forms of 

 aggregation, require in many cases to be counteracted or modified by 

 mechanical processes : thus we have frequent occasion to compress bodies 

 into a smaller space, to augment their dimensions in a particular direc- 

 tion, to divide their substance, either partially or totally, in given lines or 

 surfaces, or to destroy their general form by reducing them into more 

 minute portions ; and we may consider these subjects as principally refer- 

 able to the effects of compression, extension, penetration, division, attri- 

 tion, digging, boring, agitation, trituration and demolition. The two first 

 of these articles depend on such a change as we have examined in consider- 

 ing the strength of materials, under the name of alteration, the remainder 

 on fracture. 



The instruments peculiarly intended for compression are in general of 

 the description of presses ; and the most common act by means of a screw. 

 The friction on the screw interferes considerably with the power of the 

 machine ; but it is of use in keeping the press fixed in a situation into 

 which it has been brought by force. The screw is always turned by a 

 lever ; for without this assistance, however powerful it might be, the fric- 

 tion would render it almost useless. When great force is required, the 

 screw is made as close as is consistent with the strength of its spires. 

 Mr. Hunter's double screw may also be used with advantage, where only 

 a small extent of motion is required. The screw of a printing press or of a 

 stamping press, is, on the contrary, open, and it is caused to descend with 

 considerable momentum, the handle being loaded with a weight. Wher- 

 ever a force is so employed as to produce an impulse which acts on any 

 body, the momentum which is the result of the action of the force for a 

 certain time, is usually much more powerful than the simple pressure ; 

 the degree of its efficacy depends, however, on the degree of compressibi- 

 lity 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 roller 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 



