In Fig. 113 is shown a standard range boiler connected to the range. The water is brought into the top of the tank through the pipe _a-a_, and passing through it enters the water-back by means of the pipe _b_. After passing through the water-back the water again enters the tank through the pipes _c_ and _d_, as indicated by the arrow. The _flow pipe_ (carrying the out-going water) from the water-back may be connected with the tank at _e_, as shown dotted or in some cases the connections are made at both places. The velocity of circulation depends on the vertical height of the column of hot water and the greater height will, therefore, improve the circulation and thus increase the efficiency of the heater. The circulation of the water through the tank and heater is produced by its change in weight as the water is heated. As the hot water comes from the water-back it rises in the pipe because it is lighter in weight than the cooler water of the tank. In the case of the pipe shown dotted in Fig. 113 the longer vertical rise will give a greater upward velocity of the hot water and consequently a better circulation through the entire circuit. The construction of the water-back is shown in the small drawing. The connections are made at _b_ and _c_ as before. A division plate in the water-back causes the water flowing in at _b_ to follow the length of the heater at the bottom and return at the top as indicated by the arrow, when it is discharged at _C_. The hottest water is always at the top of the tank and the temperature grades uniformly from the hottest at the top to the coolest at the bottom. The reason for extending the pipe _a_ so far down into the tank is that the cold water may not mingle with the hot water and reduce its temperature on entering the tank. Near the top of the pipe _a_ is a small hole _f_ that is intended to prevent the water from being siphoned from the tank in case a vacuum is formed in the cold-water pipe. In this arrangement the water enters and leaves at the top of the tank. In case the supply is shut off at any time the tank is left almost full of water, because the siphoning effect cannot extend below the small hole _f_.