Chap. XXIX. ON THE CONSTRUCTION OF MACHINERY.

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workmen employed; by this means a firm buttress was formed, which resisted the inwardpressure of the earth, as progress was made in the tunnel: for a further description of theapplication of this shield, we must refer to p. 522. The brickwork which lined the bottom,sides, and arches of the tunnel, were completed up to the shield before it was pushedfarther along.

Wherever large sewers or drains are required to be made under the beds of navigablerivers, no better nor safer method can be adopted than that of the shield; the workmen areprotected by it, and a great irruption cannot take place suddenly, nor without giving amplewarning to allow them to escape.

CHAP. XXIX.

ON THE CONSTRUCTION OF MACHINERY.

When machines were formed of wood, the art of the millwrightwas required to put together the various movements, and otherportions of his work, in such a manner that they resisted every strainto which they were subjected: since the universal introduction ofiron for this purpose, he is now only required to prepare moulds forcasting them; still there are occasions when it is necessary for thecivil engineer to have his machinery constructed as formerly, and itis then important that the work be properly performed.

It is necessary that the ram of a pile-engine should be suspended

ei ^

Fig. 2906. ram.

Fig. 2907. Ram

Fig. 2905. ram.

□ .

Fig. 2908. screw

so that it falls perpendicularly in the groove prepared to direct its motion, and this hasbeen admirably and simply effected by attaching to the back of the ram two frames in theform of a H, the arms of which serve as a guide when rising or falling. Our limits will

Fig. 2909.

Fig. 2910.