598 The Trials of Light Portable Motors at Plymouth. 
charge into the clearance space of the engine. It is then ex- 
ploded by an induction spark, expands driving the piston, and 
on the return stroke is expelled. The cycle is similar to that of 
the Otto engine. A water jacket is used, as in gas engines, to 
keep the cylinder cool ; but it would rather seem from the obser- 
vations that not only is the initial explosion rather less violent 
than in a gas engine, but the -temperature of the cylinder is 
lower. 
The governing arrangement is extremely ingenious. A 
small high speed governor is connected to a taper spindle, through 
which the oil passes to the spray-maker. The rotation of this 
spindle regulates the amount of oil used. At the same time a 
throttle valve, actuated by the same governor, regulates the 
amount of air admitted to the vaporising chamber. In starting, 
a small hand pump is used to initially compress the air in the 
oil reservoir, and for a short time the vaporising chamber is 
heated by a lamp. In the first trial of the engine, the heating 
of the vaporising chamber began at ten hours and fourteen min- 
utes. At ten hours twenty-seven minutes the engine started 
without difficulty. Indicator diagrams were taken every fifteen 
minutes. The temperature of the water flowing into and out of 
the jacket, and the spring balance load, were noted every ten 
minutes. The amount of oil used was determined by weighing 
in enough oil into the oil reservoir to fill it to the same level as 
at starting. The water flowing from the jacket was weighed. 
No oil is used in lubricating the cylinder which could add to 
the fuel burned. The explosions were noticed from time to time, 
but they were perfectly regular, and the exhaust was quiet and 
apparently contained no unburnt oil. No missed explosion was 
noticed, so that the number of explosions is taken at half the 
number of revolutions. 
As in many explosive engines the efficiency falls off greatly 
when the load is reduced, it was thought desirable to make" a trial 
with half the normal load. There is an unavoidable loss of effi- 
ciency with a decreased load, because the engine friction, which 
in these small engines is considerable, is nearly constant and in- 
dependent of the load. Making allowance for this, the perform- 
ance of Messrs. Priestman's engine in the half-power trial may 
be taken to have been satisfactory. It will be seen that the 
engine friction was 075 horse-power, or 11< percent, of the indi- 
cated power in the full-power trial, and 0 - 85 horse-power, or 20 
per cent, of the indicated power, in the half-power trial. 
Messrs. Brown & Mays engine was a nominal 1 horse-power 
engine, with a cylinder 4 - 252 inches diameter and 9 inches stroke. 
It weighs empty 18] cwt., and costs 100/. The oil used was 
