Prof. Barnard on a modification of the Ericsson Engine, 241 
Assigning now to J, m, and n, the values heretofore. employed 
respectively, we shall find, by comparing (i) and (ii) with (1) and 
(2), and by comparing (1) with (II), that while the total effective 
pressures of both the mutually auxiliary engines, taken together, 
e not widely different, yet the parts which make up this total 
are remarkably unequal in the two cases. The substitutions of 
e numerical values of the letters, give the following results; a, 
as before, being taken at 22100 sq. in. 
(i) = 12a = 265200 lbs. 
(1) = 7:555a = 166977 “ 
i 4a = 88400 id 
a 
6:286a = 138920 ‘ 
Ld 
(I) = 16a=353600 “ 
(II) = 13-841a=305897 “ 
It appears from these comparisons, that, at the moment of maxi- 
mum joint effective action, one of the laboring pistons of Ericsson’s 
engines is sustaining three times as great a pressure as the other ; 
and that this is precisely the piston of which the power is applied at 
the greatest mechanical disadvantage. By the other construction, 
this inequality is much reduced. The “otal of effective action is 
also diminished at this point by the proposed plan; as it was de- 
signed to be, for the benefit of a later period of the stroke. 
_ The point of maximum resistance is likewise, when a cut-off 
is employed, the point of minimum elasticity of the air in the 
working cylinder—that is to say, the end of the stroke ; and this 
will therefore be the point of minimum joint effective pressure in 
the pair of engines. In Ericsson’s engines, we have, for the pis- 
ton finishing its stroke, 
P=at! —mat—15(1-m)a (’) 
and for that at mid-stroke, as before, a 
P=at—ami—15(1-m)a. tS, 
For an engine working without a cut-off, these expressions are 
obviously equal; but it is otherwise for one which works expan- 
sively, 
_ In the newly proposed form, the expression for the first piston 
is exactly the same as (i’). There is no escape from the neces- 
sity of encountering somewhere a resistance equal to the full ten- 
sion, t, exerted against the entire surface of the condensing piston. 
his, moreover, happens, when ¢ has run down to én the work- 
ing cylinder; that is, when the positive pressure is least. We 
have, therefore, for piston finishing its stroke, : 
And, for the piston, at mid-stroke, as before, (2) 
(oe aoe) (2’) 
Elia E © ereeatt 
Szconp Serres, Vol. XVI, No. 47.—Sept., 1853. - ol 
