MEOHANICS AND USEFUL ARTS. 17 



we have described, pierced through the plate-iron from one half to 

 three fourths of an inch in thickness. 



Some of the steam-arms or levers just described are gifted with 

 what may be termed " double thumbs," and accordingly these perforate 

 two holes at a time, or forty per minute, the round pieces of iron cut 

 out falling, at each pulsation of the engine, upon the ground, through 

 the matrix or perforation in the anvil. 



When the plates, averaging from six to twelve feet in length, by 

 above two feet in breadth, have been thus punched all around, and 

 before they are brought to the tube, they are framed together on the 

 ground, in compartments of about twenty plates each (five in length 

 and four in breadth), in order to be connected to each other by what 

 are termed covering-plates and angle-irons. 



In order to prepare the former (which are half an inch in thick- 

 ness, one foot in breadth, and about two feet long), they are heated in 

 a small furnace, when, instead of passing between rollers, they are 

 put under a stamping, or, as it is technically termed, a joggling-block, 

 which, by repeated blows, renders their surface perfectly flat; after 

 which a series of holes, corresponding in size as well as in distance 

 from each other with those in the " plates," is punched all along the 

 outer edge of each of their four sides. When thus prepared, two of 

 these small covering-plates, one on each side, are made to cover and 

 overlap the horizontal line of windage existing between the edges of 

 the plates, which, as we have stated, have been previously arranged 

 so as to touch each other ; and bolts being driven through the corre- 

 sponding holes of the three plates (the large plates lying between the 

 two covering ones), they are firmly riveted together by the process 

 we are going to describe. 



In the construction of the Britannia tubes there have been required 

 no less than two millions of bolts, averaging seven eighths of an inch 

 in diameter and four inches in length. The quantity of rod-iron con- 

 sumed for this purpose has, therefore, amounted in length to 126 miles, 

 and in weight to about 900 tons! The mode in which these legions 



^ o 



of rivets have been constructed is briefly as follows. At the western 

 end of the company's principal forging establishment there stands a 

 furnace or trough, full of pieces of rod-iron, from 3f to 4-f- inches 

 in length. As soon as, by the bellows worked by steam, they have 

 been made uniformly red-hot, a little boy picks them out one after 

 another through the furnace-door with a pair of pincers, from which 

 he drops them perpendicularly into eight moulds, each of which being 

 about three quarters of an inch*shal lower than the length of the piece 

 of iron it respectively receives, they, of course, all equally protrude 

 about that distance above the surface. They are then placed upon an 

 anvil, and the protruding portion is flattened by a hammer worked by 

 steam, so as to become at once a boll. 



As soon as each "set" of the half-inch iron plates which form the 

 sides, top, and bottom of the Britannia tubes have by a travelling- 

 crane been lifted technically termed " picked up" into their 

 place, and have been made to touch each other as closely as possi- 

 ble, a movable stage on wheels is drawn close to the outside of the 



2* 



