The modern derrick, made of steel, and operated by steam or electricity, capable of lifting tons, yet absolutely obedient to the hand of the engineer, is a really wonderful piece of mechanism. A steam-scoop, for example, excavating a gravel bank, seems almost a thing of intelligence; as it gores into the bank scooping up perhaps a half ton of earth, its upward sweeping head reminds one of an angry bull. Then as it swings leisurely about and discharges its load at just the right spot into an awaiting car, its hinged bottom swings back and forth two or three times before closing, with curious resemblance to the jaw of a dog; the similarity being heightened by the square bull-dog-headed shape of the scoop itself. Yet this remarkable contrivance, with all its massive steel beams and chains and cog wheels, employs no other principles than the simple ones of lever and pulley and inclined plane that we have just examined. The power that must be applied to produce a given effect may be calculated to a nicety. The capacities of the machine are fully predetermined in advance of its actual construction. But of course this is equally true of every other form of power-transmitter with which the modern mechanical engineer has to deal. FRICTION In making such calculations, however, there is an additional element which the engineer must consider, but which we have hitherto disregarded. In all methods of transmission of power, and indeed in all cases of the contact of one substance with another, there is an element of loss through friction. This is due to the fact that no substance is smooth except in a relative sense. Even the most highly polished glass or steel, when viewed under the microscope, presents a surface covered with indentations and rugosities. This granular surface of even seemingly smooth objects, is easily visualized through the analogy of numberless substances that are visibly rough. Yet the vast practical importance of this roughness is seldom considered by the casual observer. In point of fact, were it not for the roughened surface of all materials with which we come in contact, it would be impossible for any animal or man to walk, nor could we hold anything in our hands. Anyone who has attempted to handle a fish, particularly an eel, fresh from the water, will recall the difficulty with which its slippery surface was held; but it may not occur to everyone who has had this experience that all other objects would similarly slip from the hand, had their surfaces a similar smoothness. The slippery character of the eel is, of course, due in large part to the relatively smooth surface of its skin, but partly also to the lubricant with which it is covered. Any substance may be rendered somewhat smoother by proper lubrication; it is necessary, however, that the lubricant should be something which is not absorbed by the substance. Thus, wood is given increased friction by being moistened with oil, but, on the other hand, is made