236 Prof. Barnard on a modification of the Ericsson Engine. 
it occurs between 40 and 45 degrees from the neutral line, and 
varies very slowly, through a considerable arc: the mechanical 
advantage of the driving engine iucreasing nearly as the resist- 
ance of the opposing. During this time, the effective pressure is 
reduced below 14 pounds to the square inch. The action, there- 
fore, of the double engine, though not entirely neutralized by 
this cause, is rendered very undesirably unequal. ; 
Without going into a more minute exposition of this branch of 
the subject, it is the design of the present article to suggest a 
modification of the form of the engine, by which it is believed 
that the unequal action resulting from the use of high tension 
may be in a measure removed, so that single engines may wor 
with larger supply cylinders, without intervals in which the power 
will be zero; and double engines may carry the tension of the 
air in the reservoir to a much higher degree than is practicable at 
present, without encountering the difficulties which have been 
mentioned. 
The proposed remedy is not entirely unobjectionable itself; 
but it is apparently recommended by advantages more important 
than any objections which have yet presented themselves. An 
if its adoption shall render it possible to obtain as high power as 
has yet been reached in practice, from materially less piston sur- 
face, the most serious of these objections, which arises from an 
increase of the number of parts, and therefore of weight, will dis- 
appear. Possibly the modification may ultimately lead to a re- 
duction of wei 
That the proposition may be understood, it is proper here to 
observe, that, at the commencement of the stroke, there is an 
amount of pressure which, at a high tension of the air upon these 
huge pistons, becomes absolutely enormous. With the tension 
aimed at by Ericsson, viz. 12 Ibs. above the atmosphere, it becomes 
no less than 265000 pounds. Before the completion of the half- 
stroke, this is reduced to one-third of its value, when it remains 
constant up to the cut-off, at the two-thirds’ stroke, after which it 
rapidly diminishes, becomes zero at eighteen twenty-thirds of the 
stroke, and thence finally negative to the end. The object of the 
modification about to be suggested, is to provide a relief for | 
tively free. In order to effect this object, it is proposed to employ 
three cylinders and three pistons where Ericsson employs only 
two; and to extend over a complete double stroke the labor of 
driving each fresh charge of air from the atmosphere into the 
reservoir. 
