sum acquired by a few days' labour; yet, when this is employed in
blasting rocks, effects are produced which could not, even with the best
tools, be accomplished by other means in less than many months.

_Economy of Materials._--The worn-out saucepans and tin-ware of our
kitchens, when beyond the reach of the tinker's art, are not utterly
worthless. We sometimes meet carts loaded with old tin kettles and
worn-out iron coal-scuttles traversing our streets. These have not yet
completed their useful course; the less corroded parts are cut into
strips, punched with small holes, and varnished with a coarse black
varnish for the use of the trunkmaker, who protects the edges and angles
of his box with them; the remainder are conveyed to the manufacturing
chemists in the outskirts of the town, who employ them, in conjunction
with pyroligneous acid, in making a black dye for the use of calico
printers.

_Accumulation of Power_ arises from lifting a weight and then allowing
it to fall. A man, even with a heavy hammer, might strike repeated blows
upon the head of a pile without producing any effect. But if he raises a
much heavier hammer to a much greater height, its fall, though far less
frequently repeated, will produce the desired effect.

_Regulating Power._--A contrivance for regulating the effect of
machinery consists in a vane or a fly, of little weight, but presenting
a large surface. This revolves rapidly, and soon acquires an uniform
rate, which it cannot greatly exceed, because any addition to its
velocity produces a much greater addition to the resistance it meets
with from the air. The interval between the strokes on the bell of a
clock is regulated by this means; and the fly is so contrived, that this
interval may be altered by presenting the arms of it more or less