until the
energy is all absorbed, and a vacuum or deficit of pressure exists in the
cylinder instead of an excess of pressure. The return stroke is
accomplished by the atmospheric pressure, and the work is now done upon
the engine shaft on the return only. The method of connecting on the
return stroke while leaving the piston free on the out stroke varies, but
in many engines the principle was the same.

Barsante and Matteucci, year 1857, British patent No. 1,625, describe the
first engine of this kind, but Messrs. Otto and Langen were the first to
successfully overcome all difficulties and make a marketable engine of it.
Their patent was dated 1866, No. 434. To distinguish it from Otto's later
patents, it may be called the rack and clutch engine.

The economy obtained by this engine was a great advance upon the Lenoir.
According to a test by Prof. Tresca, at the Paris Exhibition of 1867, the
gas consumed was 44 cubic feet per indicated horse power per hour.
According to tests I have made myself in Manchester with a two horse power
engine, Otto and Langen's free piston engine consumes 40 cubic feet per
I.H.P. per hour. This is less than one-half of the gas used by the Hugon
engine for one horse power.

The igniting arrangement is a very good modification of Barnett's lighting
cock, which I have explained already, but a slide valve is used instead of
a cock.

Other engines carried out the same principle in a different manner,
including Gilles' engine, but they were not commercially so successful as
the Otto and Langen. Mr. F.H. Wenham's engine was of this type, and was
working in England, Mr. Wenham informed me, in 1866, his patent being
taken out in 1864.

The great objection to this kind of engine is the irregularity and great
noise in working; this was so great as to prevent engines from being made
larger than three horse power. The engine, however, did good work, and was
largely used from 1866 until the end of 1876, when Mr. Otto produced his
famous engine, now known as "The Otto Silent Gas Engine." In this engine
great economy is attained without the objectionable free piston by a
method proposed first by Burnett, 1838, and also by a Frenchman, Millein,
in 1861; this method is compression before ignition. Other inventors also
described very clearly the advantages to be expected from compression, but
none were able to make it commercially successful till Mr. Otto. To him
belongs the great credit of inventing a