560 Trials of Traction- Em/ines at IVolverJiainpton. 
work performed had been as much as was equal to the area con- 
tained by the black lines, less, say 2 lbs., average back pressure, 
as indicated by the space between the bottom line and the dotted 
line immediately above ; in fact, it would be found, if this diagram 
were contrasted with one taken from a jacketed cylinder, that the 
area of the unjacketed diagram representing the work done would 
be greater than that of the jacketed. Fig. 16 shows a diagram 
taken from a jacketed cylinder; and if it be laid over the other 
diagram, it will be found that the expansion-curve a g is lower 
than the expansion-curve a of Fig. 15 ; that is to say, that the 
height g e oi Fig. 16 is less than the height oi g^ g e of Fig. 15. 
It may be said, therefore, that the unjacketed engine of dia- 
gram. Fig. 15, made a greater use of the amount of steam that came 
into the cylinder than had the jacketed engine, Fig. 16; but the 
fact is, while in Fig. IG the parallelogram abed truly repre- 
sents the quantity of 105 lbs. steam which comes into that cylinder, 
the parallogram a b c d oi the diagram, Fig. 15, does not repre- 
sent it, because it does not show the quantity of 105 lbs. steam 
which came into the cylinder and was condensed in heating up 
the walls of that cylinder ; and in order to make diagram Fig. 15 
correct, there should be added to it a portion as b d h i, to show 
the steam condensed on its entering the cylinder. If this were 
done, it would be ascertained that that steam ought to have 
produced, if utilized, the whole of the area i a g^ e h, instead 
of the area b a g^ e d. The rise in the diagiam of Fig. 15, 
from g to g^, re])resents of course the re-evaporation of the con- 
densed steam. Now it is upon these facts that the utility of 
jacketed cylinders is based, and it will be seen to consist in the 
prevention of the condensation of high steam in the cylinder, 
and its re-evaporation in that cylinder as low steam. The loss 
Avhich takes place upon the outside of the jacket is one which 
may be materially diminished by proper cleading, and is a mere 
loss by conduction 'and radiation from the surface, about such as 
would take place from the surface of the cylinder itself. It must 
follow from what has been said here upon steam-jacketing, that 
to be of use the steam in the jacket should be at all times as 
high as the very highest steam employed in the cylinder ; in 
fact, it has often been proposed in large engines to jacket the 
cylinders with steam from an especial boiler kept at a higher 
pressure. If these facts were borne in mind, we should see no 
more attempts at abortive jacketing, by surrounding the cylinder 
with steam upon the engine side of the throttle-valve, that is, 
with steam attenuated by wire-drawing below the boiler pressure, 
nor with the exhaust steam from the engine ; nor if the very 
powerful effect of cooling-surface were borne in mind, and the 
fact that the temperature of steam must vary with its pressure, 
should we see instances in portable engines of waste steam-pipes 
