16 ANNUAL OF SCIENTIFIC DISCOVERY. 



while the four longest galleries, each 472 feet long, should be con- 

 structed upon wooden platforms, at high-water-mark on the Caernar- 

 von shore, and should be floated to the foot of the towers on pon- 

 toons, thence to be raised to their positions by hydraulic presses. 



II. Construction of the Tubes and Towers. A platform was at 

 once constructed of balks of timber covered with planks, for the build- 

 ing of the tubes, and near this platform, which was half a mile long, 

 were erected workshops, with the requisite forges and machinery. 

 Several wharves were built and six steam-engines procured. 700 

 men were employed on the iron work, and 800 on the stone for 

 the towers. We" will describe the construction of the various por- 

 tions. 



Plates. The wrought-iron plates which form the top, bottom, and 

 sides of the Britannia " land tubes," 230 feet in length, are, of 

 course, slighter than those required for the four, each 460 feet, which 

 overhang the stream. 



For these lonw tubes, which are of the same height and breadth 



o * <j 



as the shorter ones, the dimensions of the plates are as follows : 



For the bottom: 12 feet in length, 2 feet 4 inches to 2 feet 8 inches 

 in breadth, j 7 6 to % inch in thickness. 



For the top : 6 feet in length, 1 foot 9 inches to 2 feet inch in 

 breadth, f to f inch in thickness. 



For the sides : 6 feet to 6 feet 6 inches in length, 2 feet in breadth, 

 to f inch in thickness. 



Although these plates have been severally forged with every pos- 

 sible attention, yet, to render them perfect in thickness, they are not 

 allowed by Mr. Stephenson to be used for the tubes until each has 

 been passed by the company's snperintendant between two massive 

 iron rollers, worked by steam, which, by revolving, squeeze down the 

 pimples, that, from unequal contraction in the process of cooling, 

 often disfigure the surface of plate iron. When the plates, the lar- 

 gest of which weigh about seven hundred-weight, "have been thus ac- 

 curately flattened, they are, one after another, according to their di- 

 mensions, carried by two or more men towards one of several im- 

 mense cast-iron levers, which, under the influence of steam, are to be 

 seen from morning till night ascending and descending once in three 

 seconds. 



Beneath the short end of this powerful lever there is affixed to the 

 bottom of a huge mass of solid iron a steel bolt, which, endowed 

 with the enormous pressure of from 60 to 80 tons, sinks at every 

 pulsation of the engine, into a hole rather larger than itself, perforated 

 in a small anvil beneath. 



As soon as the laborers of the department bearing each plate ar- 

 rive at this powerful machine, the engineer in charge of it, assisted 

 by the carrying men, dexterously places the edge of the iron upon 

 the anvil in such a position that the little punch in its descent shall 

 consecutively impinge upon one of a series of chalk dots, which, at 

 4 inches from each other and 1 inch from the edge, have been 

 previously marked around the four sides of the plate ; and thus four 

 rows of rivet-holes, averaging an inch in diameter, are, by the power 



