558 
Trials of Traction- Engines at Wolverhampton. 
wasted in the unjacketed cylinders ; the excess being (in Tred- 
gold's judgment) that due to the extra size of the jacket over and 
above that of the cylinder which it enveloped. 
After so great a writer as Tredgold had fallen into this error, 
and had condemned Watt, no engineer need be ashamed to con- 
fess that he does not see how a steam-jacket may be an advantage. 
We think, therefore, we may be pardoned if we say a few words 
in order to explain the paradox of the steam-jacket, although quite 
aware that while we are doing it the bulk of the engineering 
readers of these lines do not want any information upon the 
subject. We must, therefore, be taken as simply addressing 
ourselves to those engineers who may not have studied the 
question, and to that section of our readers who, not being 
engineers, nevertheless take an interest in engineering science. 
The steam-jacket is of especial use in the expansive engine, 
and the greater the amount of expansion the greater is the need 
for and the use of the steam-jacket. 
Assume the case of an expansive (say non-condensing) engine 
without a steam-jacket. The piston has been making a stroke 
towards, say, the right-hand end of the cylinder; and in making 
that stroke the pressure of the exhaust-steam on that side of the 
piston has been 1 or 2 lbs. above the atmosphere, and the tempera- 
ture therefore practically that of boiling water. 
The metal of the cylinder sides has, so far as its interior skin 
is concerned, been cooled down to the temperature of the exhaust 
steam. In this condition of things, steam, say at 105 lbs. to the 
inch above the atmosphere, and at a temperature of 343 degrees 
(about), is admitted from the boiler, and it comes into a chamber 
the walls of which are 131 degrees lower than itself. A quantity 
of steam sufficient to supply the heat to heat up the walls of the 
chamber must therefore be at once condensed ; this is done, and 
the condensed steam remains in the form of water in the cylinder 
until the slide-valve by its motion has shut off the communica- 
tion with the boiler ; the steam in the cylinder then begins to 
expand and the pressure to be reduced. The water arising from 
the steam which was first condensed is now in contact with the 
walls of the chamber heated to a temperature due to 105 lbs., 
while the pressure in the cylinder has diminished from 105 lbs. 
gradually down to say 7i lbs. above the atmosphere ; the inevitable 
result of this is, that the water which was first condensed becomes 
re-evaporated and turned into steam to be used in the cylinder. It 
may be said that, if this is so, its power, which was lost in the act 
of condensation, will be brought back again by its re-evaporation. 
But it must be recollected that its power was lost when it was 
105 lbs. steam, and that while it is being re-evaporated at all sorts 
of intermediate pressures down to T^^lbs., the difference in effect 
