Trials of Traction-Engincs at Wolverhampton. 
557 
means for tlie insertion of a pyrometer in that part of his engine 
where the products of combustion have passed tlie last useful 
heat-absorbing surface, be it boiler or be it heater, and where 
they have not yet mingled with the waste steam. 
Steam- jachetiiig. — One of the great difficulties in writing these 
Reports is, that they are addressed to two totally different classes 
of readers : the one, the manufacturers and exhibitors of the 
very machines to which the Report relates (a class to which it 
is almost an insult to offer anything in the nature of engineering 
criticism at all, and certainly anything bordering upon the ele- 
ments of engineering) ; the other, the agriculturists who purchase 
and use these implements. The latter class of readers are not 
supposed to understand engineering as a science, and, in reporting 
for their information, one is bound therefore to enter into the 
consideration of engineering subjects in a manner which is neces- 
sarily elementary. This must be our apology for having devoted 
a page or two of this Report to the description of the steam- 
engine indicator, and of its mode of action. But we now wish 
to say a word or two upon the question of steam-jacketing engines ; 
and we are about to take the liberty of saying it quite as much 
for the benefit of the manufacturers as for that of the purchasers. 
Although some of the engines were steam-jacketed, and some 
were so constructed as to superheat their steam to a point which 
might probably enable steam-jackets to be dispensed with, others 
were not jacketed although the steam was not superheated. 
Moreover, at previous trials of engines, the Judges have noticed 
that steam-jacketing was the exception, and that in some instances 
Avhere the exhibitor has supplied jackets, he clearly did not appre- 
ciate their value. 
Some few years ago, on the occasion of a trial of stationary 
engines from the Society's boiler, an exhibitor who had provided 
his engine with a steam-jacket did not use it, because he thought 
that as he could not return the condensed steam into that boiler, 
it would be a loss to him rather than a gain. We believe that 
very many engineers — not only those engaged in the manufacture 
of agricultural engines but engineers generally — do not appreciate 
the theory upon which the benefit of steam-jackets rests ; and they 
share in the mistake of the greatest writer that probably we have 
ever had upon the steam-engine (we allude to Tredgold), who, 
clear as he was in almost every point connected with that sub- 
ject, was, in respect of this particular of steam-jacketing, in error, 
and was led from that error to condemn James Watt's practice 
of the steam-jacket as a mistake. For Tredgold, not appreciating 
the theory of the steam-jacket, held that it was a mere contri- 
vance for keeping the cylinder warm ; and that, although it did 
this, it did it by the waste of more steam than would have been 
