Review of Prof. Johnson’s Report on American Coals. 315 
boiler to a certain temperature (usually 230°) by means of a 
weighed quantity of dry pine wood. During this period no steam 
was allowed to escape. The coal was then substituted, after 
withdrawing and weighing the unburnt wood, and the process 
continued with coal alone to the end. The ashes which the re- 
quired quantity of wood would give having been determined by 
other experiments, were deducted from the whole amount af res- 
idue left. 
The following table, extracted from the Report, will give some 
idea of the observations at each trial, their variety, and the care 
with which they were made. (See table on succeeding pages 
316, 317.) 
The weight of coal consumed at each trial was generally from 
800 to 1200 pounds. Four trials were generally made upon each 
sample of coal. ‘The mode of conducting the combustion was in 
some respects varied in the different trials, chiefly with a view of 
determining the influence of such changes or modifications upon 
the efficiency of the material. One important modification in ope- 
ration, which was introduced in the experiments upon almost 
every species of coal, and to which we have not yet referred, was 
the introduction of fresh atmospheric air to the gases immedi- 
ately behind the grate. This was accomplished by placing in 
that part of the apparatus a perforated iron plate, through which 
the air from the ash-pit below could pass, and which could be 
closed by simply drawing over it another perforated iron plate. 
The effect of this modification in the manner of conducting the 
combustion is shown in the course of the experiments. . 
The tables of daily trials made upon each coal are followed by 
another, in which all the most important facts, either of observa- 
tion or deduction, are clearly tabulated. No explanation in rela- 
tion to this table, within the limits we have assigned for this re- 
view, would be so satisfactory an exhibition of its character as the 
table itself; we therefore submit one selected at hazard. Some 
things in this table may possibly not be fully comprehended by 
mere inspection, but the limits of this review do not allow us to 
enter into an explanation of them. The Report itself does this 
fully upon every point, and we therefore must refer the reader to 
that document for such farther information as may be necessary. 
(See the table inserted on pages 318, 319.) 
