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 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.