The steam engine familiarly explained and illustrated; with an historical sketch of its invention and progressive improvement; its applications to navigation and railways; with plain maxims for railway speculators. By the Rev. Dionysius Lardner ... With additions and notes by James Renwick ...

LOCOMOTIVE ENGINES ON TURNPIKE ROADS. 223
ing, to prevent the crust thus formed from accumulating;
and this operation, for obvious reasons, is attended with peculiar difficulty in tubular boilers. In the case before us,
the crust of deposited matter would gather and thicken in
the tubes c and K, and if not removed, would at length
choke them. But besides this, it would be attended with a
still worse effect; for, being a bad conductor, it would intercept the heat in its transit from the fire to the water, and
would cause the metal of the tube to become unduly heated.
Mr. Gurney of course foresaw this inconvenience, and contrived an ingenious chymical method of removing it by occasionally injecting through the tubes such an acid as would
combine with the deposite, and carry it away. This method
was perfectly effectual; and although its practical application
was found to be attended with difficulty in the hands of common workmen, Mr. Gurney was persuaded to adhere to it
by the late Dr. lWollaston, until experience proved the impossibility of getting it effectually performed, under the circumstances in which boilers are commonly used. Mr. Gurney then adopted the more simple, but not less effectual, method of removing the deposite by mechanical means. Opposite the mouths of the tubes, and on the other side of the cylinders D and H, are placed a number of holes, which, when
the boiler is in use, are stopped by pieces of metal screwed
into them. When the tubes require to be cleaned these stoppers are removed, and an iron scraper is introduced through
the holes into the tubes, which, being passed backward and
forward, removes the deposite. The boiler may be thus
cleaned by a common labourer in half a day, at an expense
of about Is. 6d.
The frequency of the periods at which a boiler of this kind
requires cleaning must depend, in a great degree, on the nature of the water which is used; one in daily use with the
water of the river Thames would not require cleaning more
than once in a month. Mr. Gurney states that with water