On Compound Engines for Agricultural Furposes. 663 
cylinder full of steam, about 94 per cent, of the power which 
would have been obtained if steam had been admitted to the 
cylinder throughout the stroke, thus economising steam and 
fuel to the extent of about 20 per cent, by even this limited 
amount of expansion. 
With two eccentric and expansion slide-valves, or other more 
complicated appliances, the steam admission may be cut off 
much earlier still in the stroke, and still greater economy may 
be thus obtained. At the Cardiff trials most of the portable 
engines cut off the steam admission at about ^ to ^ of the 
stroke, with 80 lbs. boiler pressure. 
The higher the boiler pressure, the earlier the steam may be 
cut off with advantage, and the greater the economy resulting 
from expansion. 
The gear, however, necessary for great expansion in a single 
cylinder, is always complicated in its details ; it easily gets out 
of adjustment, and it is rarely equable in its action at both 
ends of a cylinder. An engine too, with high expansion, is 
difficult to start with the load on, and as steam cools rapidly as 
it expands, some of the gain from high expansion is lost again 
by the condensation of the incoming steam in the same cylinder, 
which has just been cooled nearly to the temperature of the 
exhaust steam. 
This last difficulty is partly but not wholly met by jacketing 
the cylinder with a steam casing connected with the boiler, 
which imparts to the steam in the cylinder, as it expands and 
cools, some extra heat, and at the same time keeps the cylinder 
warm, for the next incoming " gulp " of live steam. The idea 
that an air casing or good lagging is as effective as a steam 
jacket is a decided mistake. They only reduce radiation. The 
true theory is, that the steam jacket imparts positive heat to the 
cylinder and to the steam expanding within it. 
The compound engine, by the addition of an extra cylinder, 
enables us to get all the expansion which we desire, with simple 
valve gear, and the losses from cooling are further neutralised, 
by dividing the extreme ranges of temperature between two 
cylinders. 
The steam is admitted first to the small high-pressure 
cylinder to do work against its piston, and thence is exhausted 
at a medium pressure ; then it does about an equal amount 
of work again, in the low-pressure cylinder of a larger diameter, 
being finally exhausted into the open air at a pressure slightly 
above that of the atmosphere. 
The general result is, that by adding a small amount to the 
cylinder weight, with extra piston, slide valve, and other gear 
of a simple character, we can do the same work as before, with 
VOL. XVII. — S. S. 2 Y 
