112 



WELLS'S NATURAL PHILOSOPHY. 



Fig. 97. 



•where the arm of the crank is horizontal, the power 

 from the rod acts with tlie greatest advantage, an 

 at the extremity of a lever. But when the rod 

 which communicates motion stands perpendicular 

 with the arm of the crank, as in No. 2, which is 

 the case twice during every revolution, the power, 

 however great, can exert no effect upon the resist- 

 ance, the whole force being expended in producing 

 pressure upon the axle and pivots of the crank. 

 Such a situation of the rod and the arm of the 

 crank is called the dead point, and when the ma- 

 hinery stops, as is often the case, it is said to bo 

 "set," or "caught on its center." The difficulty is 

 generally overcome by the employment of a fly- 

 wheel (§ 21), which, by ita inertia, keeps up the 

 motion. 



SECTION II. 



FRICTION. 



What propor- 253. TliG most sGrious obstacle to the per- 

 ma°h&" is fection of machinery is Friction ; and it is 

 lost by friction ? ^^gyally consiclcred to destroy one third of the 

 power of a machine. 



254. Friction is of two kinds : shding and 

 kinds of"fnv rolling. Sliding friction is produced hy the 



ticn are there ? , . , . , . ^ /> ^ , 



sliding, or dragging ot one surlace over another ; 

 rolling friction is caused by the rolling of a circular body 

 upon the surface of another.* 



Friction increases as the weight, or pressure increases, aa 

 tioTincrease ?' ^^"^ surfoces in contact are more extensive, and as the rough- 

 ness of the surfaces increase. With surfaces of the samo 

 material, friction is nearly proportional to the pressure. 



Friction diminishes as the weight or pressure is less, as the 

 tioTdimhifsh ? polish or smoothness of the moving surfaces is more perfect, 

 and as the surfaces in contact are smaller. It may also be 

 diminished by applying to the surfaces some unguent, or greasy material: 

 oils, tallow, black-lead, etc., are commonly used for this purpose ; they dimin- 

 ish friction by filling up the minute cavities and smoothing the irregularities 

 that exist upon the surface.* Oils are the best adapted for diminishing the 

 friction of metals, and tallow the friction of wood. 



• All hollies, however much they may he polished, appear rough and uneren when 

 examined with a microscope. 



