A. T. Rogers, of New York, writes:

"This problem has been put to me: 'How much coal is required to heat the water in a swimming bath containing, say 85,000 gallons of water, by hot-water circulation?' The system proposed is to connect the flow pipe from the heater at one end of the tank, near the top, and the return at the bottom at the other end thus causing all the water of the bath to pass through the heater. From data which I have obtained I have calculated that to raise this amount of water from, say 400 to 900 Fahr., would require the consumption of about 2,800 pounds of coal. Allowing a consumption of five pounds of coal per hour per square foot of grate surface, a grate containing 56 square feet of surface would be required to do the work in 10 hours. I should like to hear from the experience of others in this kind of work."

[The 85 000 gallons of water would weigh approximately 708,333 pounds. To raise the temperature of this weight of water from 400 to 90° Fahr., or through 500 Fahr., would call for the expenditure of about 708,333 X 50, or 35,416, 650 heat units. Assuming that 10 pounds of water can be evaporated by one pound of coal (good ordinary practice), it will require about 3,541 pounds of coal to do the work required in 10 hours, or at the rate of 354 pounds of coal burned per hour. Allowing, say, 10 pounds of coal to be burned per square foot of grate surface per hour, which is a fair Figure, 35.4 square feet of grate area would be required to do the work.]