182 SIMPLE MACHINES 



wood or metal together, and for lifting weights, but also for producing 

 great pressure. The letter-copying press (Fig. 184) and the vise are 

 screws used for this purpose. 



206. Friction. The law of machines (cf. 197) does 

 not turn out to be exactly true in the actual use of a 

 machine, because some of the power exerted is used in 

 overcoming friction (cf. 24). But while friction repre- 

 sents lost effort, some friction is usually necessary in order 

 that a machine may " work." Thus, a barrel must 

 "stick " slightly to an inclined plane, or we cannot roll it 

 up; a rope passed over a pulley must adhere to the rim of 

 the pulley, or the pulley wheel will not turn when the 

 rope is drawn in. We all know how difficult it is to walk 

 upon a highly polished floor, owing to the lack of friction 

 between our shoes and the floor. Also, heavy objects 

 that are to be moved horizontally are placed on rollers or 

 wheels, if possible, because rolling friction is much less 

 than sliding friction. The wheelbarrow illustrates this 

 admirably, also the use of castors on bedsteads, tables, etc. 



207. The Sailboat. Few discoveries of man have 

 helped him more in his rise to civilization than his ability 

 to travel long distances by water; this was made possible 

 largely by his discovery of the use of sails. 



We can readily understand how a boat can sail " before 

 the wind": the sail merely provides a large area for re- 

 ceiving the pressure of the moving air. The sailboat not 

 only permits us to sail in the direction of the wind, but it 

 permits us to exchange rapid motion in this direction for 

 slower motion in some other direction. No sailboat, of 

 course, can go directly against the wind. 



