FURTHER ILLUSTRATIONS OF PRINCIPLES. 101 



s a five-sided, slightly tapering piece of steel fixed into a wooden 

 handle by means of which it can be cautiously worked into the hole 

 hat is to be widened. The rimer serves not only for widening 

 holes, but also for making them properly round and smooth. It 

 is best to have a set, varying from 2 to 7 mm in thickness, for 

 iioles of different sizes. 



Of the numerous illustrations of the principles in- 

 volved in the action of simple machines, the following 

 two deserve attention. 



Fig. 72 is a small roller, thicker in the middle than 

 at the ends. Two threads are fixed to the thinner 

 ends, coiled a few times round them, and then fastened 

 to hooks which are fixed to a support. Two other 

 threads are fixed to the thicker part, coiled round it 

 in an opposite direction, and the other extremities are 

 connected by a small wooden crossbar. The roller, if 

 left to itself, moves downwards by its weight, and the 

 two outer threads are uncoiled from the cylinder, while 

 the two inner threads are wrapped on to the thicker 

 part, and in consequence of the difference of thickness 

 more thread will be wound round the larger cylinder 

 than is coiled off the smaller. In the figure, the circum- 

 ference of the smaller ends is 6 cm ; that of the middle, 

 ipart about 7 cm *5; hence when the roller makes one re- 

 volution it falls through 6 cm , while the crossbar rises 

 'through 7-5 6 = l cm '5, or one-fourth of that space. It 

 follows from the principle of equality of work that it will 

 be possible to keep the roller at rest if a certain force be 

 applied at the crossbar. Thus, if the roller weighs ICF, 

 a weight of 40 gr suspended at the crossbar will produce 

 equilibrium, and, if a larger weight be suspended, the 

 roller will be made to rise while the weight sinks. 



