INTRODUCTION TO MECHANICS. xxix 



contact of the rubbing- surfaces is so close, and the rubbing- so continual, 

 that, notwithstanding their being polished and oiled, a considerable degree 

 of friction is produced. It is a remarkable circumstance, that there is 

 generally less friction between two bodies of different substances, than of 

 the same. It is on this account that the holes in which the spindles of 

 watches work, are frequently made of jewels ; and that when two cog- 

 wheels work in one another, the cogs of the one are generally made of 

 wood, and of the other of metal. 



There are two kinds of friction ; the one occasioned by the sliding 

 of the flat surface of a body, the other by the rolling of a circular 

 body. The friction resulting from the first is much the most consi- 

 derable ; for great force is required to enable the sliding body to over- 

 come the resistance which the asperities of the surfaces in contact op- 

 pose to its motion, and it must be either lifted over, or break through 

 them ; whilst, in the other kind, the friction is transferred to a smaller 

 surface, and the rough parts roll over each other with comparative facility ; 

 hence it is, that wheels are often used for the sole purpose of diminishing 

 the. resistance of friction. When, in descending a steep hill, we fasten 

 one of the wheels, we decrease the velocity of the carriage by increasing 

 the friction, that is to say, by converting the rolling friction of one of 

 the wheels into the dragging friction ; and when casters are put to the 

 legs of a table the dragging is converted into the rolling friction. 



The great fly-wheel which is frequently attached to steam-engines and 

 other large machines, acts in the first instance as a heavy weight to im- 

 pede their free uncontrolled motion. However paradoxical this mode of 

 improving machinery may appear, it is nevertheless of great advantage. 

 The motion of a machine is always more or less variable, owing to the 

 irregularity both of the power which works it, and of the resistance which 

 it has to overcome. Whether the power consists in wind, water, steam, or 

 the strength of animals, it cannot be made to act with perfect regularity, 

 nor can the work which the machine has to perform be always uniform. 

 Yet in manufactures, and most cases in which machinery is employed, 

 uniformity of action is essentially requisite, both in order to prevent injury 

 to the machine, and imperfection in the work performed. A fly-wheel, 

 which is a large heavy wheel attached to the axis of one of the principal 

 wheels of the machinery, answers this purpose, by regulating the action 

 of the machine : by its weight it diminishes the effect of increased action, 

 and by its inertia it carries on the machine with uniform velocity when the 

 power transiently slackens; thus by either checking or impelling the 

 action of the machine, it regulates its motion so as to render it tolerably 

 uniform. 



There is another circumstance which we have already noticed as 

 diminishing the motion of bodies, and which greatly affects the power of 

 machines : this is the resistance of the medium in which a machine is 

 worked. All fluids, whether of the nature of air, or of water, are called 

 mediums ; and their resistance is generally proportioned to their density: 

 for the more matter a body contains, the greater the resistance it will 

 oppose to the motion of another body striking against it. It is therefore 

 more difficult to work a machine under water than in the air. Jf a machine 

 could be worked in vacua, and without friction, it would be perfect ; but 

 this is unattainable. A considerable reduction of power must, therefore, 

 be allowed for the resistance of the air. 



