An obvious defect which such a boiler presents is the difficulty of removing from it any deposite or incrustation, which may collect between the sheets of copper so closely and intricately connected. Dr. Church proposes to effect this, when it is required, by the use of an acid, which will combine readily with the incrustation, and by which the boiler may therefore be washed. This method of cleansing boilers was recommended by Dr. Wollaston to Mr. Gurney, who informed me, however, that he found that it was not practicable in the way in which boilers must commonly be used. I apprehend, also, that the spaces between the sheets of copper, in Dr. Church's boiler just described, will hardly permit the steam bubbles which will be formed to escape with sufficient facility into the steam-chest; and being retained in that part of the boiler which is exposed to the action of the fire, the metal will be liable to receive an undue temperature. I have, however, seen this engine working, and its performance was very satisfactory. (113.) Various other projects for steam carriages on common roads are in various degrees of advancement, among which may be mentioned those of Messrs. Maudslay and Field, Col. Macerone, and Mr. Russell of Edinburgh; but our limits compel us to omit any detailed account of them. CHAPTER XII. STEAM NAVIGATION. Propulsion by paddle-wheels. -- Manner of driving them. -- Marine Engine. -- Its form and arrangement. -- Proportion of its cylinder. -- Injury to boilers by deposites and incrustation. -- Not effectually removed by _blowing out_. -- Mr. Samuel Hall's condenser. -- Its advantages. -- Originally suggested by Watt. -- Hall's _steam saver_. -- Howard's vapour engine. -- Morgan's paddle-wheels. -- Limits of steam navigation. -- Proportion of tonnage to power. -- Average speed. -- Consumption of fuel. -- Iron steamers. -- American steam raft. -- Steam navigation to India. -- By Egypt and the Red Sea to Bombay. -- By same route to Calcutta. -- By Syria and the Euphrates to Bombay. -- Steam communication with the United States from the west coast of Ireland to St. John's, Halifax, and New York.