in at atmospheric pressure.  In its upward stroke the piston was free to move; but in its downward stroke it was connected with a ratchet, and the partial vacuum formed after the explosion beneath the piston, together with its own weight in falling, operated through a rack, and caused rotation of the flywheel.  This engine (which, in an improved form, uses only about 20 cubic feet of gas) is still largely employed, some 1,600 having been constructed.  The great objection to it was the noise it produced, and the wear and tear of the ratchet and rack arrangements.  In 1876 the Otto-Crossley silent engine was introduced.  As you are aware, it is a single-acting engine, in which the gas and air are drawn in by the first outward, and compressed by the first inward stroke.  The compressed mixture is then ignited; and, being expanded by heat, drives the piston outward by the second outward stroke.  Near the end of this stroke the exhaust-valve is opened, the products of combustion partly escape, and are partly driven out by the second inward stroke.  I say partly, for a considerable clearance space, equal to 38 per cent. of the whole cylinder volume, remains unexhausted at the inner end of the cylinder.  When working to full power, only one stroke out of every four is effective; but this engine works with only 18 to 22 cubic feet of gas per horse power.  Up to the present time I am informed that about 18,000 of these engines have been manufactured.  Several other compression engines have been introduced, of which the best known is Mr. Dugald Clerk’s, using about 20 feet of Glasgow cannel gas.  It gives one effective stroke for every revolution; the mixture being compressed in a separate air-pump.  But this arrangement leads to additional friction; and the power measured by the brake is a smaller percentage of the indicated horse power than in the Otto-Crossley engine.  A number of gas engines—­such as Bisschop’s (much used for very small powers), Robson’s (at present undergoing transformation in the able hands of Messrs. Tangye), Korting’s, and others—­are in use; but, so far as I can learn, all require a larger quantity of gas than those previously referred to.

[Illustration:  OTTO ATMOSPHERIC GAS ENGINE.]

[Illustration:  CLERCK’S GAS ENGINE, 6 HORSE POWER.]

[Illustration:  OTTO-CROSSLEY GAS ENGINE, 16 H.P.

               Consumption 17.6 cubic feet of 16-candle gas per
               theoretical horse power per hour.

               Average pressure, 90.4 x constant, .568 theoretical
               horse power per pound = 50.8 theoretical horse power.]

[Illustration:  ATKINSON’S DIFFERENTIAL GAS ENGINE, 8 H.P.]