to the rate at which it was being drawn from
the boiler. Sediment was found to lodge in the lowermost point of the
boiler at the rear end and the exposed portions cracked off at this
point when subjected to the furnace heat.

[Illustration: No. 4]

No. 4. A plain cylinder, carrying the water line at its center and
leaving the upper half for steam space, was substituted for the nest of
tubes forming the steam and water space in Nos. 1, 2 and 3. The sections
were made as in No. 3 and a mud drum added to the rear end of the
sections at the point that was lowest and farthest removed from the
fire. The gases were made to pass off at one side and did not come into
contact with the mud drum. Dry steam was obtained through the increase
of separating surface and steam space and the added water capacity
furnished a storage for heat to tide over irregularities of firing and
feeding. By the addition of the drum, the boiler became a serviceable
and practical design, retaining all of the features of safety. As the
drum was removed from the direct action of the fire, it was not
subjected to excessive strain due to unequal expansion, and its
diameter, if large in comparison with that of the tubes formerly used,
was small when compared with that of cylindrical boilers. Difficulties
were encountered in this boiler in securing reliable joints between the
wrought-iron tubes and the cast-iron headers.

[Illustration: No. 5]

No. 5. In this design, wrought-iron water legs were substituted for the
cast-iron headers, the tubes being expanded into the inside sheets and a
large cover placed opposite the front end of the tubes for cleaning. The
tubes were staggered one above the other, an arrangement found to be
more efficient in the absorption of heat than where they were placed in
vertical rows. In other respects, the boiler was similar to No. 4,
except that it had lost the important element of safety through the
introduction of the very objectionable feature of flat stayed surfaces.
The large doors for access to the tubes were also a cause of weakness.

An installation of these boilers was made at the plant of the Calvert
Sugar Refinery in Baltimore, and while they were satisfactory in their
operation, were never duplicated.

[Illustration: No. 6]

No. 6. This was a modification of No. 5 in which longer tubes were used
and over which the gases were caused to make three passes with a view of
better economy. In addition, some of the stay