Prof. Barnard on a modification of the Ericsson Engine. 243 
against some negative pressure, may, with similarly enlarged sup- 
ply cylinders, acquire much greater efficiency. 
Without enlarging upon this point, it will be sufficient to take 
the expression for mean pressure already given, and substitute in 
it larger values for m. If m=1, and /=1, n being 1'8, as before, 
we shall obtain a mean pressure of 70200 Ibs. and, with nine 
revolutions, and a 6 foot stroke, a horse-power of 230. 
If 2 be put =3, m and n remaining the same, the horse-power 
rises to 1340. But with this adjustment, the periodical resistance 
becomes intolerable. Indeed. it is obvious that when m is equal 
to 1, no single engine can work with a cut-off at all, without a 
heavy fly ; nor with a short cut-off, even with the aid of such a 
regulator. For when m=1 and J also =1, the power and resist- 
ance are exactly balanced at the end of the stroke.* But double 
engines may have great power, with a condensing as large 
as the working cylinder, and without objectionable negative pres- 
sure. Thus, put m=1 and /=, and the horse-power of a single 
engine rises to 390. 
The maximum effect is not attained, however, with m=1. 
The expression for mean pressure, viz., 
P= 15a (mn —1)+ 15am ( (1—m) hl /—hl mn) 
becomes a maximum (n and / being constant) when 
hl m= (n—1) —hln+(1-—n) hl 
* In Ericsson’s sogines; if m==1, the power is zero throughout half the stroke, even 
when working without a cut-off. In fact, this paralysis of the power 
out much more than half the stroke, in consequence of the heat developed by com- 
Pression, as is shown further on. 
