recently be seen in its original, still more in modified form, in
certain back-quarters of civilisation. A stream, guided by a sluice, was
made to play upon four vertical paddle-blades, attached to a shaft which
they caused to revolve, and which moved a millstone, resting upon
another through which it passed. It was a primitive mill, which
superseded the still more primitive hand-mill, or quern; and I myself
have seen it at work in the Shetland Islands, and even the north of
Scotland, though it is now done away with even there, still more farther
south, and its place supplied and its work done by overshot and
under-shot wheel-gear, and improved machinery attached, of less or more
complexity. One of the most recent improvements is the Turbine, a sort
of Barker's mill; it is of great power, small compass, and acts under a
good fall with a minimum expenditure of water-power.

Passing from the consideration of water as a motive power in its natural
state, I ask you to notice briefly the gigantic force it can be made to
develop under the action of heat. In its normal form the power of water
is due, as I have said, to its incompressibility; in the state of
vapour, to which it is reduced by heat, its power is due to the counter
force of expansion. It was when confined as a state prisoner in the
Tower of London that the Marquis of Worcester began to speculate on the
possibilities of steam, though he little dreamed of its more important
applications, and the incalculable services it might be made to render
to the cause of humanity. Suddenly, one day, his musings in his solitude
were interrupted by the rattling of the lid of a kettle, which was
boiling away on the fire beside him, when, being of a philosophic vein,
he commenced to inquire after the cause; and he soon reasoned himself
into the conclusion that the motive power lay in the tension of the
vapour, and that the maintenance of this must be due to successive
additions of heat. The thought was a seed sown in a fit soil, for it led