The Trials of Light Portable Motors at Plymouth. 601 
The heat value, as determined from the chemical composi- 
tion, is therefore 204 evaporation units, or 19,700 thermal units 
per lb. The uncertainty as to the heat value to which Mr. 
Wilson alludes is due to the fact that the heat of formation of 
the hydrocarbons in the oil is not known. It is almost certain 
that the heat value deduced from the carbon and hydrogen 
should be reduced a little to allow for this. Consequently the 
heat value will be taken in the following calculations at 19,000 
thermal units. 
Economy of cost of fuel in petroleum engines. — The Brown 
& May engine may be set aside as not having been in perfect 
working order during the trial. It remains then to compare the 
results of the Priestman full-power trial with results obtained in 
motors of other types. For the purpose of such a comparison it 
may be assumed that the very best large condensing steam 
engines use 1^ lb. of Welsh coal per indicated horse-power per 
hour. Taking Messrs. E. R. & F. Turner's engine, to which 
the Second Prize in the class of steam engines was awarded, as 
representing a fairly good example of small non-condensing steam 
engines of about the same power as the petroleum engine, the 
consumption is 8-461 lb. of Welsh coal per indicated horse- 
power per hour. Gas engines working with Dowson gas pro- 
ducers are known to work with about 1*3 lb. of coal per indicated 
horse-power per hour. It is obvious that the Priestman engine 
works with about as much fuel as the very best large steam 
engine, with about one-eighth as much fuel as a small non-con 
densing engine, and with an economy about as great as that 
of a gas engine with Dowson gas producer. The comparison 
can, however, be made more accurately if we assume, on the 
best data available, values for the cost and calorific value of the 
fuel. 
Data assumed. Welsh steam coal, 24s. a ton or 0 - 1286d. 
per lb. Heat value, 14,600 Th. U. per lb. 
Coal gas. — In Professor Kennedy's trial of the Otto gas 
engine 20-76 cubic feet were used per indicated horse-power. 
The heat value of the gas was 626 Th. U. per cubic foot, 
and the cost of the gas may be taken at 3s. per 1,000 cubic 
feet. 
Petroleum. — The oil used in the Priestman engine cost 
0-802cZ. per lb., and its heat value will be taken at 19,000 Th. U. 
per lb. 
With these data we get the table on the following page. 
There is no doubt that the figures in this table show the 
performance of the Priestman oil engine to have been a re- 
markably good one. Taking actual weight of fuel used per indi- 
VOL. I. T. S. — 3 R R 
