The Trials of Light Portable Motors at Plymouth. 597 
air, and then exploded in a cylinder. The action of these 
engines is therefore very similar to that of a gas engine, with 
the difference, however, that the fuel used is much more port- 
able. It may be said at once that the working of the Priest- 
man engine was altogether satisfactory, and it worked with an 
economy of fuel almost unprecedented. So far as could be 
judged from a trial extending altogether over nearly six hours, 
the action of the engine was faultless, and, in spite of the high 
price of the fuel used, it is probable that it may in many cases 
supersede steam engines for small powers and for intermittent 
work. 
The other engine, exhibited by Messrs. Brown & May, is 
clearly at present in a much less satisfactory condition. Pos- 
sibly it was in part an unfortunate accident that at the formal 
trial there was great difficulty in starting it. It was certainly 
in any case much less economical of fuel than the Priestmau 
engine. Without wishing in any way to conclude that this 
engine cannot be made a satisfactory and useful one, but judg- 
ing it merely as it behaved on the trial run, it did not appear 
desirable or just to pass so favourable a verdict on its perform- 
ance, as would be implied by the award of a prize. 
Messrs. Priest/man' 8 engine (First Prize 30/.). — This was a 
nominal 4^ horse-power engine, weighing empty 47-g- cwt., or 
about the same as a single cylinder portable steam engine of the 
same power. The engine is described by the makers as a port- 
able oil engine (Priestman & Hume's patent), with oil cistern, 
vaporiser, air pump, and electric igniting apparatus, mounted 
on wheels and axles, with shafts. 
The cylinder is 8-499 inches diameter, the stroke 12 inches, 
and the normal speed 180 revolutions per minute. The clear- 
ance space is given as 403-6 inches. 
The fuel used on the trial was a mineral oil selected for use 
in the Northern Lighthouses, and sold as Broxbourne oil. It is 
stated to cost 6^d. per gallon. 
The density of the oil, determined by Mr. C. J. Wilson, was 
0-8101, so that it costs 0"802cZ. per lb. 
The action of the engine is this. The oil reservoir is 
supplied with a small amount of compressed air by the air 
pump. Under the action of the air pressure the oil is driven 
through a carefully regulated nozzle into a vaporising chamber, 
kept hot by the exhaust from the engine. The liquid oil is split 
up into spray as it enters the vaporising chamber, and apparently 
evaporates instantly. In the forward movement of the piston, 
a supply of vapour and air is drawn into the cylinder from the 
vaporising chamber. The return of the piston compresses the 
