Fig. 70.
Fig. 71.
118. Other Illustrations.—We see the tendency of fluids to be on the same level in other ways. In a coffee-pot the liquid has the same level in the spout as in the vessel itself, whatever may be its position, as seen in Fig. 70 (p. 86). If it be turned up so far that the level of the fluid in the vessel is higher than the outlet of the spout, the fluid runs out. If two reservoirs of water be connected together the water will stand at the same height in both, whatever the distance between them may be. So, also, in the aqueduct pipes that extend from a reservoir, the water will rise as high as the surface of the water in the reservoir itself. If the outlets of the pipes be lower than this level the water will run from them, as in the case of the coffee. The cause of these and similar facts is the same as that of the level surface in vessels and reservoirs—the action of gravitation. This may be made plain by Fig. 71. Let the figure represent a vessel with divisions of different degrees of thickness, these divisions, however, not extending to the bottom of the vessel. Water in this would stand at the same level in the different apartments, just as it would if the vessel had no such divisions, as represented. This is simply because the attraction of the earth acts upon the water in the same way with the divisions as without them. And you can see that it will make no difference whether these divisions be thick or thin, or whether the apartments be near, as you see here, or far apart, as they are when branch pipes extend from a reservoir. A branch pipe may be considered as having the same relation to the reservoir, as one of the narrow apartments in the figure has to the rest of the vessel. The result is not at all affected by either the size or form of the tubes that may be connected with a common reservoir—a fluid will stand at the same height in all.
Fig. 72.
Fig. 73.
Thus we have, in Fig. 72, tubes of various size and shape, a b c d e, connected with a reservoir, r, and if water be poured into one of them it will rise to the same height in all, just as in the different apartments of the vessel represented in Fig. 71. A man once thought that he had gained the great desideratum, perpetual motion, by a vessel constructed as in Fig. 73. He reasoned in this way: If the vessel contain a pound of water, and the tube only an ounce, as an ounce can not balance a pound, the water in the vessel must be constantly forcing that in the tube upward. It therefore must constantly run out of the outlet of the tube, and as it flows into the vessel the circulation must go on, and the only hindrance to its being a perpetual circulation would be the evaporation of the water. He was confounded when he found, on pouring water into the vessel, that it stood at precisely the same level in the vessel and the tube.