Trials of Traction- Engines at Wolverhampton. 531 
the nominal power of such engines. It was stated that no pur- 
chaser would be contented without twice the nominal power, 
would not be surprised at thrice, would not object to lour times, 
and that if he lived in the neighbourhood of a sea-port and had 
experience of the performances of marine engines, even as much 
as six times the nominal power might be looked for by him. 
This year the Society has gone further in leaving to exhibitors 
full liberty to exercise their skill and judgment. They have not 
only given perfect freedom in respect of dimensions for a nominal 
horse-power, but they have also permitted the exhibitor to try 
the engines upon the dynamometer at any power he pleased not 
below the nominal, and to work at any pressure of steam he 
pleased, taking the precaution, however, to obtain from each 
exhibitor proper drawings of his boilers and a declaration of the 
steam-pressure at which he intended to work, and then ascer- 
taining by proof that the boilers were competent to sustain double 
that pressure, and by calculation that they were safe at three 
times that pressure. 
Furthermore, the proportions of the different parts being thus 
given by the exhibitor,enables the Judges in their report to tabulate 
them, so that the purchaser of an engine can see when he buys a 
certain nominal engine of A whether, taking heating surface and 
grate surface, as well as cylinder dimensions, into consideration, 
be is buying a more or less powerful machine than the engine of 
B of the same nominal power. 
On this occasion, also, the Society have acceded to the advice 
offered to them by their engineers, and by the past and present 
Judges in former reports, and have caused the amount of water 
evaporated by each exhibitor when on trial at the brake to be 
recorded, the oil and tallow consumed also to be recorded ; and 
further, they have directed each engine to be fitted with proper 
cocks for applying the indicators supplied by the Society, so as 
to enable the Judges to take indicator diagrams at the time the 
engines were being tried upon the brake. 
In the Oxford Report it was thought that it might be of 
interest to the readers of the Journal were the Judges to give a 
brief description of the nature and principles of the brake so long 
used by the Society for the purpose of ascertaining the perform- 
ance of the engines offered for trial. It may now perhaps be well 
to give (also briefly) a description of the nature, the principles, 
and the uses of the steam-engine indicator, now for the first time 
employed by the Society. 
The Indicator. — The steam-engine indicator was invented by 
the great James Watt. It is an implement by means of which 
an engine Is caused to write on a piece of paper an accurate 
record of the performances that take place within the cylinder; 
