1062
THEORY AND PRACTICE OP ENGINEERING.
Hook I I.
is then drawn up to be discharged : as the work advances an additional rod is introduced,by means of screws, which can be readily turned by a wrench, and taken to pieces as oftenas is required.
The operation of boring is often resorted to when it is necessary to examine several bedsof earth, and to discover their nature. The engineer should on all occasions be careful tomake himself acquainted with the foundations previous to his operating in any way uponthe surface of the soil, and this is still more important when the constructions are under
Fig. 1626.
BORING INSTRUMENTS.
water : when the soil to be examined is so situated, a hollow cylindrical pile is drivendown, within which the boring rods can with facility turn ; at the bridge at Moulins suchan operation was carried on to the depth of nearly 50 feet, and at Ambleteuse to 80 feet:at the latter place a trough was used made of planks, in lengths of 6 or 7 feet, and placedin succession one upon the other.
The borers used in France are of several kinds, and well adapted for every variety ofsoil : Ilericart de Thury, who was inspector-general in 1810, contrived two very simplemethods of uniting the boring-rods ; one, by means of a fork, the other by the screw :forking was preferred, although it occupied a greater time to perform.
A more economical method of boring was practised by M. Sellow, near Saarbruck inChina , who used only a heavy bar of iron about 6 feet 4 inches in length, and 4 inches indiameter, armed at its lower end with a cutting chisel, and surrounded with a hollowchamber, to receive through-valves, and bring up the detritus of the perforated stratum; itwas suspended by a rope that passed over a pulley, and as the rope was pulled, the iron rodin mounting received a circular motion, which was sufficient in its descent to turn round
the chisel. After the chamber was full the whole apparatus was lifted, and the earthemptied out, when the chisel was again dropped, and the work continued: a thousandfeet have in this manner been bored by the Chinese , who perforate holes in the earth18 inches in diameter, and several hundred feet in depth, to give ventilations to theirmines.