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 produce, as
it will show at once from whence our power is derived. "A pint of water
may be evaporated by two ounces of coal; in its evaporation it swells to
216 gallons of steam, with a mechanical force equal to raising a weight
of thirty-seven tons one foot high." A pound of coal in a locomotive
will evaporate about five pints of water, and in their evaporation these
will exert a force equal to drawing two tons on a railway a distance of
one mile in two minutes. A train of eighty tons weight will take 240
passengers and luggage from Liverpool to Birmingham and back, each
journey about four and a quarter hours; this double journey of 190 miles
being effected by the combustion of one and a half tons of coke, worth
about twenty-four shillings. To perform the same work by common road
would require twenty coaches, and an establishment of 3800 horses, with
which the journey would be performed each way in about twelve hours,
stoppages included. So much for the advantages of steam.

The Romans are supposed to have had some knowledge of the power of
steam. Among amusing anecdotes, showing the knowledge the ancients had
of steam, it is told that Anthemius, the architect of Saint Sophia,
lived next door to Zeno. There existed a feud between them, and to annoy
his neighbour, Anthemius had some boilers placed in his house containing
water, with a flexible tube which he could pass through a hole in the
wall under the floor of Zeno's dwelling; he then lit a fire, which soon
caused steam to pass through the tube in such a quantity as to make the
floors to heave as if by an earthquake. But to return. We next come to