It was discovered some years ago that if the vapor of gasoline or naphtha was mixed with a definite quantity of air, and a light was applied to the mixture, an explosion would result.  Modern science uses the force of such exploding gases for the accomplishment of work, such as running of automobiles and launches.

In connection with the gasoline supply is a carburetor or sprayer, from which the cylinder C (Fig. 130) receives a fine mist of gasoline vapor and air.  This mixture is ignited by an automatic, electric sparking device, and the explosion of the gases drives the piston P to the right.  In the 4-cycle type of gas engines (Fig. 130)—­the kind used in automobiles—­the four strokes are as follows:  1.  The mixture of gasoline and air enters the cylinder as the piston moves to the right. 2.  The valves being closed, the mixture is compressed as the piston moves to the left. 3.  The electric spark ignites the compressed mixture and drives the piston to the right. 4.  The waste gas is expelled as the piston moves to the left.  The exhaust valve is then closed, the inlet valve opened, and another cycle of four strokes begins.

[Illustration:  FIG. 130.—­The gas engine.]

The use of gasoline in launches and automobiles is familiar to many.  Not only are launches and automobiles making use of gas power, but the gasoline engine has made it possible to propel aeroplanes through the air.

CHAPTER XVIII

PUMPS AND THEIR VALUE TO MAN

181.  “As difficult as for water to run up a hill!” Is there any one who has not heard this saying?  And yet most of us accept as a matter of course the stream which gushes from our faucet, or give no thought to the ingenuity which devised a means of forcing water upward through pipes.  Despite the fact that water flows naturally down hill, and not up, we find it available in our homes and office buildings, in some of which it ascends to the fiftieth floor; and we see great streams of it directed upon the tops of burning buildings by firemen in the streets below.

In the country, where there are no great central pumping stations, water for the daily need must be raised from wells, and the supply of each household is dependent upon the labor and foresight of its members.  The water may be brought to the surface either by laboriously raising it, bucket by bucket, or by the less arduous method of pumping.  These are the only means possible; even the windmill does not eliminate the necessity for the pump, but merely replaces the energy used by man in working it.

In some parts of our country we have oil beds or wells.  But if this underground oil is to be of service to man, it must be brought to the surface, and this is accomplished, as in the case of water, by the use of pumps.

An old tin can or a sponge may serve to bale out water from a leaking rowboat, but such a crude device would be absurd if employed on our huge vessels of war and commerce.  Here a rent in the ship’s side would mean inevitable loss were it not possible to rid the ship of the inflowing water by the action of strong pumps.