When water is converted into steam, any foreign matter which may be combined with it is disengaged, and is deposited on the bottom of the vessel in which the water is evaporated. All boilers, therefore, require occasional cleansing, 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 chemical 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. _Wollaston_, 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 backwards and forwards, removes the deposite. The boiler may be thus cleaned by a common labourer in half a day, at an expense of about 1_s._ 6_d._ 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 of the most unfavourable description, once a fortnight would be sufficient. (107.) In the more recent boilers constructed by Mr. _Gurney_, he has maintained the draught through the furnace, by the method of projecting the waste steam into the chimneys; a method so perfectly effectual, that it is unlikely to be superseded by any other. The objection which has been urged against it in locomotive engines, working on turnpike roads, is, that the noise which it produces has a tendency to frighten horses.