245. =Friction in Machinery.=--You have seen, as we have proceeded, that the Mechanical Powers, though thus named, do not generate power. So far from this, there is really a loss of power in their use, chiefly from friction. In raising a weight, for example, directly by the hand, there is no loss from this cause; but if you use a pulley you have the friction of the cord upon it, and a loss of power in proportion to the amount of friction. In some cases the loss of power from this cause is so great as to call for a considerable variation from such calculations as we have made in this chapter in regard to the relations of power and weight in machinery. In the operations of the screw friction has a great influence in diminishing the power of the instrument. 246. =The Real Advantages of the Mechanical Powers.=--If there is then no saving, but a loss of power in tools and machinery, what, let us inquire, are their advantages? If one man can do alone by the aid of some instrument what would otherwise require the exertion of many men, though he be slow in doing it, yet it is a great advantage. Thus one man can with a lever move a stone which perhaps it would require thirty men to move without it, and though it take him thirty times as long, it saves him the trouble of getting a company of men to help him. So if a man can raise his goods by a wheel and axle to the upper loft of his store, though he raise them slower than several men would lift them directly by ropes, it is an advantage to him, as it saves the hiring of a company of laborers. A few men by a capstan can raise an anchor which could be raised without it only by a large company of men. Another advantage often is that there may be intervals of rest in applying the force without any loss. This is obvious in the case of the pulley, but still more so in the case of the screw. It is friction in both these cases which enables the workman to rest. It saves to him all that he has gained by opposing any tendency to slip back. We see the same thing in the wedge. When this is driven into wood, it remains because it is prevented from returning by the friction of the wood against its sides. It is the same cause which holds a nail in its place, and opposes any effort to draw it out. In driving the wedge the workman can have as long intervals as he pleases between his blows, because friction saves all that is gained. This effect is very well exemplified in the capstan, Fig. 170. It requires but little exertion of the man who sits there to hold the rope, because the few turns of it around the axle prevent its slipping easily.