A unit of heat is the amount of heat required to raise the temperature of a pound of water one degree, the water being at the temperature of maximum density, about 39.1° Fahrenheit. The table below shows the number of units of heat required to convert one pound of water, at the temperature of 32°, into steam of various pressures.

In a non- condensing engine, if the exhaust steam escapes directly into the atmosphere, it carries off most of the heat that was previously imparted to it by the coal in the furnace. This may be illustrated by an example.

Suppose the steam is admitted into the cylinder of an engine at. a pressure of 90 pounds per square inch, and exhausted at a pressure of 1 pound above the atmosphere, and that the temperature of the feed - water is 70. If the feed water had been at 32°, it will be seen that each pound of water would have required 1183 units of heat to convert it into steam of 90 pounds pressure ; but since the temperature of the feed was 38° above 32°, 38 units less, or 1145, will be necessary. The exhaust steam carries away 1148 less 38, or 1110 units of the heat that has been imparted to the steam by the coal, so that about 96.94 per cent of the heat is thrown away. Now, it costs money to heat this water, and the steam-user who pays $6.50 a ton for coal, which converts 15, 000 pounds of water into steam, might make up an account somewhat after this manner: "After evaporating 15, 000 pounds of water at a cost of $0.00043+ per pound, I allowed 14, 541 pounds to escape into the air without rendering me any equivalent, and only utilized 459 pounds in my engine; so that really I pay at the rate of $0, 014+ for every pound of water used."

There are many steam-users to-day who could readily make up an account somewhat like the preceding.

Now suppose that the steam-user, being convinced of the folly of paying for coal to raise steam which is blown away without doing any good, attaches a heater to his exhaust-pipe, and so raises the temperature of the feed-water to 200, instead of 70°, as before. If this is done, and the heater is a good one, which does not increase the back pressure, each pound of water requires 130 units less for its evaporation, and each pound of exhaust-steam carries away 130 units less than before; and the steam-user can make up his account anew, as below:

"One ton of coal now evaporates about 16, 900 pounds of water, at a cost of $0.00038 per pound, and 600 pounds of this water are utilized in the engine; so that I now pay $0.0108 for each pound of water that produces useful effect."

If he had an engine of 100 horse-power, using 30, 000 pounds of steam a day, and working 300 days in a year, he would find that the difference in his coal bill, before and after the change, would be the difference between $3870 and $3420, or $450. B.