ived--as, for instance, in a common pan--yet if the vessel is
closed or shut up at the top, you will find that the vapour acquires
such a degree of elastic force, that, if not allowed to escape by fair
means, it would soon make a way or vent for itself by bursting whatever
vessel it was contained in. Steam is thus highly elastic, but when
separated from the fluid out of which it is generated, it does not
possess a greater elastic force than the same quantity of air. If, for
example, a vessel is filled with steam only at 212 deg., it may be brought
to a red heat without fear of bursting; but if water is also in the
vessel, each additional quantity of heat causes a fresh quantity of
steam to be generated, which adds its elastic force to that of the steam
already in the vessel, till the constantly accumulating force at last
bursts the vessel.

This elastic vapour is called steam, and it is by this that that most
beautiful machine, the steam-engine, is driven. As you all know, by this
vapour or air--for it is invisible till it loses part of its
heat--enormous power is obtained in a small compass, and the labour of
man reduced to nothing compared with former ages. Many men laboured to
perfect machinery to be worked by this vapour of water, and many came
near the mark; but it remained for the great Watt, at the Soho Works,
Birmingham, to bring the engine to its useful and working state, for
though discovered as a motive power 120 B.C., it was yet reserved for
this truly great man to be what may be termed the inventor of the
steam-engine.

In 120 B.C., Hero of Alexandria made a machine to be driven by steam. It
consisted of a hollow sphere into which the steam was admitted;
projecting from the sphere were two arms, from which the steam escaped
by three holes on the side of _each_ arm opposite to that of the
direction of its revolution, which, by removing the power from off the
one part of _each_ arm, caused it to revolve in the direction opposite
to that of the hole that allowed the steam to escape. This kind of
engine has been for some years in use by Mr. Ruthven of Edinburgh. There
are others who have followed very closely on Hero's plan in more ways
than one; for instance, it is the common Barker's mill, though with this
difference, that his mill is driven by water instead of steam: Avery,
also, made a steam-engine almost exactly the same. I may here, perhaps,
just be allowed to mention what a little water and coal will pr