xxviii INTRODUCTION TO MECHANICS. 



knife is really a very fine saw, and therefore acts best when used like that 

 instrument. 



The screw, which is the last mechanical power, is more complicated 

 than the others (fig. 37). It is composed of two parts, the screw and 

 the nut. The screw S is a cylinder, with a spiral protuberance coiled 

 round it, called the thread ; the nut N is perforated to contain the screw ; 

 and the inside of the nut has a spiral groove, made to fit the spiral thread 

 of the screw; just like the lid of a box which screws on. The handle 

 which projects from the nut is a lever, without which the screw is never 

 used as a mechanical power. The nut, with a lever L attached to it, is 

 commonly called a winch. The power of the screw, complicated as it 

 appears, is referable to one of the most simple of the mechanical powers, 

 the inclined plane. If a slip of paper be cut in the form of an inclined 

 plane, and wound round a pencil, which will represent the cylinder, it 

 will describe a spiral line corresponding to the spiral protuberance of the 

 screw (fig. 38). The nut then ascends an inclined plane, but ascends it 

 in a spiral instead of a straight line. The closer the thread of the screw 



Fig. 38. 



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the more easy is the ascent, but the greater are the number of revolutions 

 the winch must make ; so that we return to the old principle, what is 

 saved in power, is lost in time. The power of the screw may be increased, 

 also, by lengthening the lever attached to the nut; it is employed either 

 for compression or to raise heavy weights. It is used in cider and wine 

 presses, in coining, book-binding, and for a variety of other purposes. 



All machines are composed of one or more of the six mechanical powers 

 we have examined. One more remark must be made relative to 

 them, which is, that friction in a considerable degree diminishes their 

 force. Friction is the resistance which bodies meet with in rubbing 

 against each other. There is no such thing as perfect smoothness or 

 evenness in nature. Polished metals, though they wear that appearance, 

 more than any other bodies, are far from really possessing it ; and their 

 inequalities may frequently be perceived through a good magnifying glass. 

 When, therefore, the surfaces of the two bodies come into contact, the pro- 

 minent parts of the one will often fall into the hollow parts of the other, and 

 occasion more or less resistance to motion. In proportion as the surfaces 

 of bodies are well polished, the friction is diminished; but it is always 

 considerable, and it is usually computed to destroy one-third of the power 

 of a machine. Oil or grease is used to lessen friction : it acts as a polish 

 by filling up the cavities of the rubbing surfaces, and thus making them 

 slide more easily over each other. It is for this reason that wheels are 

 greased, and the locks and hinges of doors oiled. In these instances the 



