the
rising column becomes all steam, but the quantity or weight circulated
will attain a maximum when the density of the mingled steam and water in
the rising column becomes one-half that of the solid water in the
descending column which is nearly coincident with the condition of half
steam and half water, the weight of the steam being very slight compared
to that of the water.

[Illustration: Fig. 6]

It becomes easy by this rule to determine the circulation in any given
boiler built on this principle, provided the construction is such as to
permit a free flow of the water. Of course, every bend detracts a little
and something is lost in getting up the velocity, but when the boiler is
well arranged and proportioned these retardations are slight.

Let us take for example one of the 240 horse-power Babcock & Wilcox
boilers here in the University. The height of the columns may be taken
as 4-1/2 feet, measuring from the surface of the water to about the center
of the bundle of tubes over the fire, and the head would be equal to
this height at the maximum of circulation. We should, therefore, have a
velocity of 8(4-1/2)^{.5} = 16.97, say 17 feet per second. There are in
this boiler fourteen sections, each having a 4-inch tube opening into the
drum, the area of which (inside) is 11 square inches, the fourteen
aggregating 154 square inches, or 1.07 square feet. This multiplied by
the velocity, 16.97 feet, gives 18.16 cubic feet mingled steam and water
discharged per second, one-half of which, or 9.08 cubic feet, is steam.
Assuming this steam to be at 100 pounds gauge pressure, it will weigh
0.258 pound per cubic foot. Hence, 2.34 pounds of steam will be
discharged per second, and 8,433 pounds per hour. Dividing this by 30,
the number of pounds representing a boiler horse power, we get 281.1
horse power, about 17 per cent, in excess of the rated power of the
boiler. The water at the temperature of steam at 100 pounds pressure
weighs 56 pounds per cubic foot, and the steam 0.258 pound, so that the
steam forms but 1/218 part of the mixture by weight, and consequently
each particle of water will make 218 circuits before being evaporated
when working at this capacity, and circulating the maximum weight of
water through the tubes.

[Illustration: A Portion of 9600 Horse-power Installation of Babcock &
Wilcox Boilers and Superheaters Being Erected at the South Boston,
Mass., Station of the Boston Elevated Railway Co. This Compan