314 Review of Prof. Johnson’s Report on American Coals. 
was so disposed as to determine the temperature of the air as it 
entered the grate, after having been made, by the construction of 
the stack, to pass entirely around the two sides of the boiler and 
under the ash-pit and main fire-place. Other thermometers were 
so placed as to determine the temperature of the air and gases as 
they escaped into the chimney after combustion, that of the water 
in the supplying cistern, and that of the steam and water in the 
boiler. The coals were measured and weighed in charges, that 
is, in a box containing exactly two cubic feet. Care was taken 
to reduce them all to the size best adapted to their combustion. 
They were charged regularly, so as to keep them as nearly as pos- 
sible in a uniform state on the grate. During the experiment a 
portion of each coal was carefully dried by means of an apparatus 
prepared for this purpose, to determine the amount of hygrometric 
moisture. The rate and manner of combustion were carefully 
observed, and the ashes, clinker, &c. weighed and preserved for 
subsequent analysis. By an arrangement exhibited in Plate II 
of the Report, a portion of the gases, atmospheric air, &c. of the 
chimney, after passing through the ignited fuel, was collected 
and analyzed. The soot deposited on the sides of the flues and 
other passages was also collected, weighed, and submitted to the 
same chemical investigation. 
The air intended for the support of combustion entered an 
opening below the ash-pit, and passing thence through air cham- 
bers on each side of the boiler, so as to absorb the heat radiated 
from this body, entered the fire by a passage from the back of the 
stack, directly under the flue below the boiler. After traversing 
the fire, the gases and other products of combustion passed under 
the whole length of the boiler, returned through it by the two 
interior flues before mentioned, and communicated with one of 
the flues leading directly to the chimney. 'This however was 
susceptible of being closed by a damper, in which case the gases, 
&c. passed by another series of flues entirely around and outside 
of the boiler below the level of the water line, and then escaped 
into the chimney. By this latter arrangement, so perfect was the 
absorption of the heat generated by the fuel, that the gases on 
entering the chimney were rarely more than 60° or 70° hotter 
than the steam in the boiler, and often much less. 
One trial or set of observations generally occupied about twen- 
ty four hours. It was commenced by heating the water in the 
tal 
