Prof. Barnard on a modification of the Ericsson Engine. 235 
since a large component of the resisting pressure will be thrown 
upon the crank and shaft. Nor is the power thus expanded by 
one engine to relieve the other to be regarded as a loss, since in 
the theoretical estimate of the power of either, the whole resist- 
ance is counted as negative. - 
Still the reduction of the joint power of both engines to zero, 
or nearly to zero, at any point, is undeniable, especially when it 
is considered that this is a state of things resulting from an ac- 
tual increase of the mean pressure. The great evil resulting 
from the use of a short cut-off, is inequality of action, and the 
advantage of resorting to this expedient must be practically 
limited, unless a remedy can be found for this difficulty. 
To illustrate this fact, let us take an example. Putting m=3, 
n=2, and [=3, we shall have at/=20a, and amt+15a (1—m) 
=22ia; an excess of resistance nearly equal to three pounds on 
the square inch of the entire surface of the working cylinder. 
But z cannot equal 2. In practice, it will, probably, rarely exceed 
1:8; and will more probably (a point to be considered further on) 
fall below 1:6. Put n=1°8, and /=3%. Then, at’/=18a; and 
amt + \5a (1—m) =23a; the resistance exceeding the power of 
the engine by 5a, which, putting a=22100 sq. inches, as in 
Ericsson’s engines, = 110500 pounds. 
ower of the companion engine, at this time, (taking it at 
mid-stroke) will be =at=27a ; and the resistance the same as in 
the former case, viz.=23a; giving an effective pressure of only 
4a, and leaving a balance of resistance against both engines = 
22100 pounds. 
If we would ascertain at what point of the stroke, in either en- 
gine, working as above, equilibrium will occur between pressure 
and resistance, we have only to make the resistance 23a which is 
t ‘ 
constant for more than half the stroke, equal to wai or t’ = 232; 
18 
or 23218; and og” wee 
Now, if the connecting rod be not more than four times the 
length of the crank, we may easily show that the power of a 
single engine, worked as here supposed, will entirely fail, while 
the angle between the crank and the line of the centre, or line 0 
neutral effect, is still more than 70°, in the pushing stroke, and 
not much less in the pulling. At this distance, should the com- 
panion engine be called into help, it expends a power greater 
than the resistanee it overcomes. But as this is only the initial 
point of the resistance, no serious task is imposed on the compan- 
ion, until the stroke is considerably farther advanced. — 
The point at which the tax upon the companion will become 
a maximum, will vary with the position of the cut-off, and with 
the proportions of the parts of the engine. In the case supposed, 
