OF COAL AND RATE OF EVAPORATION. 17 
in this case was similar in construction and setting to the 
previous three small boilers, but was 10 feet long. The sup- 
plementary boiler was 4 feet 6 inches long, standing on end 
as seen in the drawing. It was worked for a lengthened 
period to heat the feed water made to pass through it, with 
the constant result that the feed water was heated vari- 
ously from 170° to 180°. 
The draught went directly under the main boiler, and was 
made to impinge against the side of the supplementary 
boiler and to pass round it and under it. Where the 
draught so impinged, the surface remained free from soot. 
The average result was as follows, allowing in the calcu- 
lations one increment of heat to raise the temperature of 
the water from 60° to 212°, and five increments to boil it 
off : 
Heat acquired by principal or working boiler, 100... 87 
* supplementary cp 15... 13 
100 
It appears from a careful set of experiments made in 
this neighbourhood, under the inspection of another ob- 
server, that when means are employed to keep the supple- 
mentary heating surface free from soot, a still more favour- 
able result is obtained, viz: 
Heat acquired by water in working boilers, 100 ....7 8.7 
ei by water in heating pipes, 27.4... 21.3 
100.0 
In the last experiments upwards of one-third of a mile 
of 4-inch piping was in use, which received the escaped 
heat from six large boilers, each 42 feet long. Six boilers, 
assisted by the supplementary heating surface, were found 
in practice to do the work which had previously required 
eight boilers, without such assistance. It should be no- 
ticed that in neither case was the principal boiler giving 
more than 7.5 pounds of water for the pound of coal 
VOL. XV. D 
