TWO:To believe a fact is not to learn it, in the sense that these terms may be applied to mechanical knowledge; to believe a proposition is not to have a conviction of its truth; and what is meant by learning mechanical principles is, as remarked in a previous place, to have them so fixed in the mind that they will involuntarily arise to qualify everything met with that involves mechanical movement. For this reason it has been urged that learners should begin by first acquiring a clear and fixed conception of power, and next of the nature and classification of machines, for without the first he cannot reach the second.A cubic inch of water, by taking up a given amount of heat, is expanded to more than five hundred cubic inches of steam, at a pressure of forty-five pounds to the square inch. This extraordinary expansion, if performed in a close vessel, would exert a power five hundred times as great as would be required to force the same quantity of water into the vessel against this expansive pressure; in other words, the volume of the water when put into the vessel would be but one five-hundredth part of its volume when it is allowed to escape, and this expansion, when confined in a steam-boiler, exerts the force that is called steam-power. This force or power is, through the means of the engine and its details, communicated and applied to different kinds of work where force and movement are required. The water [33] employed to generate steam, like the engine and the boiler, is merely an agent through which the energy of heat is applied.
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