and definite path for the circulation, there will be no change of direction however severe the forcing.

It is very clear, therefore, that, in the Babcock & Wilcox boiler, the conditions are almost ideal for a perfect circulation, and experience has shown this to be the fact. In the tests of the "Wyoming's" boiler with oil fuel, the unprecedented rate of evaporation of almost 16 pounds per square foot of heating surface was maintained without any difficulty, and subsequent examination showed that no part of the boiler had been injured or distorted in the least degree. Without perfect circulation, such a performance is impossible.

STEAM-PROPERTIES AND LAWS OF GENERATION

W

HEN water is converted into steam it has first to be heated to a certain definite temperature which is called the boiling point. This temperature equals 212 degrees Fahrenheit for the ordinary pressure of the atmosphere (14.7 pounds above vacuum), but as the pressure is increased the boiling point increases, although at a decreasing ratio, until at 500 pounds above vacuum it equals 467.3 degrees Fahrenheit. As the water rises in temperature, it absorbs heat at the rate of one B. T. U. for each degree Fahrenheit. This is known as the heat of the liquid, or sensible heat, as it may be shown by means of a thermometer.

After reaching the boiling point, the further addition of heat transforms the water into steam without increasing its temperature. The heat thus absorbed is called the heat of vaporization, or "latent heat," and cannot be shown by any instrument for measuring temperatures. The latent heat decreases as the pressure increases, it being about 970 British thermal units per pound at atmospheric pressure, and about 762 at 500 pounds pressure above vacuum.

It will be seen, therefore, that the temperature of steam normal to its pressure, is the same as of the water at the boiling point, and also that the total heat in steam consists of two parts; first, the heat contained in the liquid at the boiling point, and second, the heat of vaporization. Or, in other words, the total heat is the sum of the sensible heat and the latent heat.

The total heat increases slightly as the pressure increases, being 1150.4 British thermal units per pound at atmospheric pressure, and 1210 British thermal units at 500 pounds.

The density of steam increases with the pressure, and varies as the 17th root of the 16th power. Its weight per cubic foot may be found by the formula w = .003027p, where p .003027p, where p = the pressure above vacuum. The results are correct within 1 per cent. up to 250 pounds pressure.

Saturated steam cannot be cooled except by lowering its pressure, any cooling effect being compensated for by some of the steam being condensed and giving up its latent heat. Neither can steam in direct contact with water be heated above the normal temperature corresponding to its pressure, providing there is an opportunity for free transference of heat; the only effect of the addition of more heat being to evaporate more water. If there is no outlet for the additional steam formed, both the pressure and the temperature will be increased. When steam is removed from contact with water, it may be heated above the normal temperature corresponding to its pressure. It is then called superheated.

The table on page 81 gives the properties of saturated steam at various pressures.

Pressures below the atmosphere, or partial vacuum, are often expressed in inches (of mercury). The following table gives the temperature and pressure of steam corresponding to various vacua.

OWNERS:

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STEAM PACKET "RAINIER POLLARD & DODGE, SAN FRANCISCO, CAL. BABCOCK & WILCOX BOILERS, 900 INDICATED HORSE-POWER

WEIGHT OF WATER AT TEMPERATURES ABOVE 200° FAHR.

WATER-THE MEASUREMENT OF HEAT

Water has a greater capacity for absorbing heat than any other known substance-bromine and hydrogen excepted. For this reason and from the fact that it is so commonly found in nature, and can be easily handled in experimental work, it has been adopted as the standard substance for measuring the quantity of heat.

Two distinct heat units are used in practice-calories and British thermal units. The latter, usually designated by the letters B. T. U., is the quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit. The calorie is the quantity required to raise a kilogram of water one degree centigrade, and is equal to 3.958 British thermal units.

The heat-absorbing capacity, or, as it is called, the specific heat of