294. =The Steam-Engine.=--As compressed or condensed air has great power by its elasticity, as seen in the air-gun, § 164, so also has condensed steam. It is steam condensed, and endeavoring, therefore, in proportion to its condensation, to expand itself, which constitutes the moving force of the steam-engine. The steam is generated in a boiler, having, like the boiler of Papin's digester, a valve with a weight attached to it. This valve is called a safety-valve, because when the steam has reached a certain degree of condensation it lifts the valve, and, as some of the steam escapes, such an increase of pressure as would occasion an explosion is prevented. The expansive force of steam in a boiler is estimated in pounds by the weight on the valve, and hence the common expression that there are so many pounds of steam on. But the boiler is only the generator of steam, and it remains to show how the steam is used in moving machinery. This is done by allowing the steam to pass from the boiler into a cylinder, and then move a piston back and forth by its expansive force. The manner in which it does this may be made clear by the diagram, Fig. 210 (p. 231). Let _e_ be a piston in a cylinder, _f_, which has four openings, _a_, _b_, _c_, and _d_. These all have valves. The steam is supplied from the boiler to the cylinder through _a_ and _c_, and makes its escape from _b_ and _d_. Suppose, now, the piston is near the bottom of the cylinder, as represented. The valve at _a_ is now opened that steam may enter to push up the piston, and the valve at _b_ shuts that the steam may not escape. At the same time, that pressure may be taken off from the upper surface of the piston, _d_ opens that the steam may escape, and _c_ shuts that none may enter. When the piston is to be forced downward all this is reversed--_c_ opens to admit the steam, _d_ shuts to prevent its escaping; and below, _b_ is opened to let the steam escape, and _a_ is shut to prevent any from entering. This is the plan of what is called the high-pressure engine. The low-pressure engine differs from it in having the steam, as it escapes from the cylinder, pass into water to be condensed. The latter requires less pressure of steam to work it, and therefore is the safest. The manner in which the motion of the piston is made to work various kinds of machinery I need not stop to explain, especially as exemplifications of it may be seen in every quarter. 295. =Communication of Heat.=--Heat has a constant tendency to an equilibrium. If therefore any warm substance be in the neighborhood of one which has less heat, a flow of heat from the former toward the latter takes place. Now this communication of heat occurs in three different ways, called Convection, Conduction, and Radiation. I will speak of each of these separately.