The trials made by Mr. Witz with the motor represented in Fig. 3 gave the following results, deduced from an experiment of 68 hours.  The figures relate to one effective horse power, measured with the brake upon the shaft of the motor.

  Consumption of anthracite. 516 grammes.
       " " coke. 96 "
  Consumption of water for the injection
    of steam. 0.487 liters. 
  Consumption of water for cooling
    the cylinder. 50.0 "
  Consumption of oil for lubricating
    the cylinder. 3.74 grammes. 
  Consumption of grease. 0.45 "
  Consumption of gas reduced to
    0 deg.  C. and to 760 mm. 2,370 liters.

This last figure will appear very high, but the fact must not be lost sight of that it is a question of poor gas, the net cost of which varies between one and two centimes per cubic meter, and the calorific power of which is but 1,487 heat units per cubic meter of constant volume, and supposing the steam condensed.  This combustion of 612 grammes of combustible per effective horse hour is remarkable, and fully shows what may be expected of the gas motor supplied by a gas generator in putting to profit certain improvements that will hereafter be possible, such, for example, as the lightening of the movable parts of the motor, the bettering of its organic rendering (now quite feeble), the use of better oils, the reduction of the consumption of water, the superheating of the steam injected into the gas generator, etc.

A well constructed steam engine, carefully kept in repair and as much improved as it is possible to make it, would certainly consume twice as much coal to produce the same quantity of effective work, say at least 1,200 grammes per horse hour.  But, as has been objected with reason, it does not suffice to compare the figures as to the consumption of fuel in order to institute a serious comparison between the steam engine and the motor using poor gas.

The gas generator requires the use of English anthracite, while a steam boiler is heated with any kind of coal.  The prices of unity of weight are therefore very different.  Moreover, the gas motor necessitates an immense amount of water for the washing of the gas and the cooling of the cylinder, through circulation in the jacket.  It is well to keep this fact in view.  On another hand, the lubrification of the cylinders requires a profusion of oil whose flashing point must be at a very high temperature, else it would burn at every explosion and fill the cylinder with coom.  Such oil is very costly.