2137. In order to develope the principles on which a hot-water apparatus acts, we may select the simple case of two vessels placed on a horizontal plane, with two pipes to connect them; the vessels being open at the top, and the one pipe connecting the lower parts of the vessels, and the other their upper parts. If the vessels and pipes be filled with water, and heat be applied to the vessel A, in fig. 630., the effect of heat will expand the water in the vessel A; and its surface will, in consequence, rise to a higher level a a, the former general surface being b b. The density of the fluid in the vessel A will decrease in consequence of its expansion; but as soon as the column (c d) of fluid above the centre of the upper pipe is of a greater weight than the column (f e) above that centre, motion will commence along the upper pipe from A to B; and the change this motion produces on the equilibrium of the fluid will cause a corresponding motion on the lower pipe from B to A; and, in short pipes, the motion will obviously continue till the temperature be nearly the same in both vessels; or, if the water be made to boil in A, it may also be boiling hot in n; because ebullition in A will assist the motion. If there be sufficient service of pipe for heating the atmosphere, or object required, a reservoir like B is not necessary to the motion of the water; a simple bent pipe, as in fig. 631., being all that is essential to motion; the reservoir B, in fig. 630., being only for the purpose of reserving a mass of hot water after the fire has gone out. These two modes, which are essentially the same, clearly illustrate the principle on which hot water is circulated in level pipes.