Fan. 19, 1882] 
the distinguished specialists who have taken it up. In 
_ the paper which the writer read recently before the Insti- 
tute of Mechanical Engineers at Manchester, and which 
has already been reproduced in the engineering journals 
(see Engineering, vol. xxxii., No. 829), a sufficient explana- 
tion of his views was given upon the merits of the use of 
steam locomotives upon tramways compared with com- 
pressed air. The objections to steam were based principally 
upon its failure to comply with the necessary conditions 
of street traction in the matter of freedom from smell and 
dirt, and also on account of the excessive cost incurred by 
the maintenance of small high-pressure boilers and ma- 
chinery. Nosuch objections can be urged against the use 
of compressed air, as compared with electricity, because in 
both cases there is nothing to give trouble or annoyance 
from the residual products. In the one case the air escapes 
in its original purity to the atmosphere from whence it 
was derived, and in the other a still more subtle trans- 
ference of force occurs, in which the conversion of one 
form of energy into another is all that takes place in 
order to effect the object aimed at. The overhead wire, 
in the Siemens system, which is the stage at which the 
invention at present stands, is a disadvantage as com- 
pared with a self-moving car in which no such obstruction 
1s necessary to its working. Overlooking this objection 
to the rival system which may possibly be overcome by 
the use of accumulation of electric force in the vehicle 
itself, the point upon which the success of both must 
ultimately turn is that of their comparative economy. At 
present there are no figures to hand that can satisfac- 
torily decide the question. In both cases a stationary 
engine is a necessary adjunct in order to supply a source 
of energy, and the future of both hinges (1) upon the 
comparative cost of the plant, and (2) upon the percent- 
age of useful work which can be obtained from the use 
of compressed air and electricity respectively. These 
questions can only be answered by the trial of both upon 
a commercial scale, but it may safely be said in the | 
meantime that there is nothing to lead to the conclusion 
that compressed air will appear to a disadvantage either 
as regards the necessary outlay in machinery or in the 
percentage of useful work to be obtained from it as 
compared with electricity. 
The conditions which effect the useful effort exerted 
by a steam-engine through the intervening medium of a 
permanent elastic fluid such as air, employed as the 
ultimate vehicle of the original force upon a piece of 
mechanism, are first the loss from friction of the com- 
pressing apparatus ; second, the loss represented by the 
difference between the temperature of the air as freshly com- 
pressed without radiation, and the temperature of the air 
as used in the second engine. These may be spoken of as 
the primary losses of energy. The secondary losses are ; 
first, the friction of the secondary engine; and secondly, 
the losses arising from its inability to utilise the whole of 
the force contained in the air as compressed and cooled. 
Now the theoretical losses arising from these various 
causes are all easily determined, with the exception of that 
arising from the defects of the secondary engine, and this, 
which amounts to more than all the rest put together, not 
only varies in each separate case, but may be fairly 
looked upon as being capable of indefinite reduction by 
discoveries and improvements in the apparatus itself. 
With regard to the fixed losses: the one which occurs 
from the loss of heat due to ccmpression and subsequent 
cooling is one that can be restored under circumstances of 
peculiar economy, as there is perhaps no condition in the 
whole range of physics which lends itself so readily to the 
economical conversion of heat into work as raising the 
temperature of an elastic fluid under compression and 
making use of it at a corresponding pressure. It must 
be remembered, however, that what we are dealing with 
in practice is not so much the saving of every heat unit of 
the original supply for the purpose of producing a 
NATURE 
267 
theoretical result and a beautiful experiment, as bringing 
the gross expense of the fuel used in the original steam- 
boiler to a point that leaves a sufficient margin as com- 
pared with horse traction, and in such a manner as not to 
interfere with the convenience of passengers. Thewriter has 
already in actual practice brought this gross sum per mile 
for fuel to }¢. when coal is used costing Ios.aton, a com- 
mon enough price in districts where tramways are in use. 
Now in attempting to reduce the cost of fuel to a smaller 
fraction of a penny than 3d. per mile run, it occurred to 
him that the effort should be made first in the direction in 
which the greatest loss occurred. This is certainly to be 
found in the defects of the secondary engine if an ordinary 
reciprocating steam-engine is employed, and an explana- 
tion of the writer's work in adapting it to the use of com- 
pressed air will be found in the paper already referred to. 
The result of his experience has gone to show that it is 
hopeless to obtain an economical result from reciprocating 
engines as at present arranged for the use of steam, with- 
out some special appliances such as he has adopted for 
making use of the ever-varying rates of expansion neces- 
sary in the case ofa self-moving car. By reason of the 
additional apparatus required for re-heating the air result- 
ing in grave inconvenience, and effecting an economy of 
perhaps not more than one-fifteenth of a penny per mile 
in fuel, he has not as yet included a heating appliance 
in the arrangements, and strong arguments would require 
to be brought to bear upon him before he determined 
upon doing so. The importance of introducing a heating 
apparatus would turn more upon what might by gained by 
adding to the capacity of a self-moving air-car with the 
view of making it capable of overtaking a particular journey 
for which the cold air was insufficient, than upon a mere 
question of economy, but even in this case he believes it 
would be more convenient and economical to add to the 
quantity and pressure of the air in the receivers than to 
make use ofa separate heating appliance to obtain the same 
result. 
Compressed air as a locomotive power is represented 
by three different systems, known respectively by the 
names of their inventors. All of them are more or less 
protected by patents, and taking the dates of the patent 
specifications as the standard of priority, the writer’s 
stands first upon the list. The other two are known as 
Mékarski’s and Beaumont’s. ‘The writer is the only one 
of the three who has made public in this country, other- 
wise than by patent specifications, the scientific work 
which he has overtaken, and the exact principles upon 
which his engines have been constructed. Before Col. 
Beaumont took out a patent at all he had driven in the 
writer’s car and examined it, but as he has departed from his 
original specification the writer has had no means of com- 
paring the efficiency of the engines, as recently constructed, 
with his own. On the occasion of his reading the paper 
at Manchester already referred to, a letter from Col. 
Beaumont was read by M. Bergeron, in which it was 
stated that the engine now running at Stratford used 10 
cubic feet of air per mile at 1000 lbs. pressure per square 
inch, or 666 cubic feet at atmospheric pressure. This 
efficiency is more than 50 per cent. less than the writer’s 
car, without allowing for the loss of power arising from 
the use of a heating apparatus, and the higher initial 
pressure of 66 as compared with 26 atmospheres to begin 
with. If this statement is correct the writer’s views with 
regard to a moderate pressure and avoiding the use of a 
heating apparatus, except when absolutely necessary, are 
fully confirmed. 
A heating apparatus, and reducing the initial pressure 
of the air by means of what is known as a reducing valve, 
are essential elements of the Mékarski system, but the 
engine would require to be considerably modified before 
it could comply with the requirements of the Board of 
Trade in this country. 
The experiments which are now being made by the 
