somewhat great diameter, so that a small space is included between the two concentric cylindrical surfaces. This space being filled with water, the fire is surrounded by a vast number of thin cylindrical shells of water, the exterior surfaces of which are exposed to the action of radiant heat, while the interior surfaces receive heat from the air which has passed from the fuel, and is carried off into the atmosphere.
While the subdivision of the water in its exposure to the fire is effected by Dr. Church, by reducing it to thin cylindrical shells, the same end is attained by Mr. Hancock, by arranging it in thin flat plates. His boiler consists of a number of thin plates of iron, placed side by side, at a distance of about an inch asunder. The water is contained between every alternate pair of plates, whilst the fire acts between the intermediate ones. It will be seen that in each case a small quantity of water exposes a very extensive surface to the fire. Mr. Hancock's arrangement, however, is said to have obvious defects. Its form being that of flat planes, exposed to a bursting force at right angles to them, is that which of all others is least conducive to strength; and although, from peculiar circumstances attending this boiler, the fact of its bursting may not be attended with danger, yet its liability to such an accident must be attended with great inconvenience, and cannot be regarded otherwise than a most fatal defect. Another defect, not less important is, that a large portion of the metal exposed to the action of fire contains steam and not water, a circumstance which should never be permitted in any boiler,—but which is utterly destructive in boilers exposed to extremes of temperature and pressure. The boiler of Dr. Church seems not to be liable in the same degree to these objections;
