There are all sorts of mills: some go by water, undershot or overshot; but if the millpond is dry, or the stream runs low, they come to a standstill. They want help, they must have water, to go on. Next there are steam-mills, which make a great noise and do a great deal of work; but they want coals and water too: if both are not brought to them, they stop and can do nothing. And then there are wind-mills; but everybody knows that wind-mills, though they do stand on the tops of hills, in spite of their great long arms stuck out, are of no use if the wind does not blow.