Agricultural Steam-engines at Newcastle. 
671 
Boiler steam, unlike a true gas, exists only in a " saturated " 
condition, in other words, it carries a load of water, so delicately 
adjusted in amount to the temperature of the steam itself that, 
' upon the latter dropping ever so slightly some of the burden 
in question is thrown down, or " condensed," and this continues 
until a new adjustment has been made between the lower heat 
of the steam and the lesser burden of water which, under the 
new circumstances, it is able to carry. There is, further, a 
constant, although not regularly progressive, relation between 
the temperature and pressure of steam, pressure increasing with 
the temperature — the more rapidly the greater the pressure, and 
vice versa. 
In the " Simple " expansive engine, steam enters the cylinder 
at or about boiler pressure, and is shortly afterwards "cut off" 
by the action of the expansion valve. It then expands in 
accordance with " Marriotte's Law," the pressure halving as 
the volume doubles, until the end of the stroke, when it is dis- 
charged at a pressure slightly exceeding that of the atmosphere, 
and at a temperature therefore of rather more than 212°. But, 
since the temperature of steam diminishes with its pressure, a 
contraction due to cooling must, under these circumstances, 
take place in the cylinder, which contraction, acting in concert 
with the expansion, further diminishes pressure, so that, unless 
the steam receives heat during its expansion, the pressure will 
diminish even more rapidly than is demanded by Marriotte's 
law. 
Now in a cylinder, which is necessarily made of metal (one 
of the best conductors of heat), condensation, followed by 
another evil, re-evaporation, have both full play. In the case of 
a cylinder, unjacketed, and supplied with stenm at, say 100 lbs. 
pressure, this, entering at a temperature of 327°, is rejected at 
a temperature of 212°, and the temperature of the cylinder itself 
must therefore lie somewhere between these two limits. Under 
these circumstances, every incoming puff of steam is cooled and 
partially condensed from the very outset of the stroke, while 
the resulting water of condensation is, in turn, re-evaporated 
from the walls of the cylinder ; both processes involving dissi- 
pation of heat-energy. 
This state of things is modified, but not remedied, by the use 
of a steam-jacket, which checks condensation and re-evaporation 
in the cylinder by transferring these operations to the jacket 
itself; an arrangement which is advantageous only because 
pressure is constant in the jacket, and variable in the cylinder. 
In any case, it must cost coal to keep up the temperature of a 
vessel filled, as the cylinder is, with a gas now hot, and now 
comparatively cold. 
