220 



LECTURE XIX. 



ON M01>ES OF CHANGING THE FORMS OF BODIES. 



I HE corpuscular forces by which bodies retain their peculiar forms of aggre- 

 gation, 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 aug-ment their dimensions in a particular direction, 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 referable to the effects of compression, 

 extension, penetration, division, attrition, digging, boring, agitation, tritura- 

 tion and demolition. The two first of these articles depend on such a change 

 as we have examined, in considering 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 ma- 

 chine; but it isof usein 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 friction would render it al- 

 most 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. Wherever a force is so employed as to produce an im- 

 pulse which acts on any b.ody, the momentum, which is the result of the 

 action of the force for a certain- time, is usually much more powerful than the 



