1064

THEORY AND PRACTICE OF ENGINEERING.

Hook II.

falling in of the earth; and where anywater is encountered, then attention mustbe paid to the best method of preventingits injury to the work.

After the shaft has been sunk to its re-quisite depth, the several timber stages thathave served the uses of the workmen areleft, or others provided, to which all therequisite pipes and machinery are attachedthat serve to pump up the water or raisethe material. The two sections exhibit theupper and lower portion of the shaft of oneof the deepest copper mines in Cornwall ,with the position of the ladders for theworkmen to 'descend, and the pumps forraising the w'ater, which with one lift andone piston often raise it 150 feet. Themain rod gives motion to the pistons ofthe different lifts of the pumps, and iseither impelled by a steam-engine or water-wheel : the pipes are of iron generally,though wood is sometimes employed.

Machines for measuring the relativestrength of men and animals are usuallycalled Dynanometers; one invented byI.evoy, of the Academy of Sciences atParis, consisted of a metal tube about 12inches long, which was placed vertically ona stand : the tube contained a spiral spring,having above it a graduated shank termi-nating in a globe ; when this was pressedinto the tube, the force used was measuredby a scale, which was graduated on theshank.

Regniers Dynanometer resembles a com-mon graphometer, the principal part ofwhich instrument is a steel spring bent inthe form of an ellipsis ; it should be pro-perly tempered, and well welded, andcovered with leather, to prevent injury tothe hands when used. This spring isrepresented by A A', B B', formed by twoequal plates united at the ends by roundedhalf-rings. The dimensions of this springvary according to the tension required, orthe weight to which it is applied.

The dynanometer used to ascertainhuman strength weighs little more thantwo pounds, and serves to measure a thou-sand times that weight; its total lengthis about 12 or 13 inches, and its greatestbreadth, as measured in the middle of thetwo arcs, is 2-2 inches, and the least breadthat the extremity of these arcs is ^ of aninch. The thickness of the arcs at theircentres is nearly 2 inches, and its height,which decreases from the centre towardsits ends, from ^ to -j^ of an inch . the chords of the two arcs are 6-4 inches. This length,added to that of the two derai-rings, gives for the total length of the dynamometer12 or 13 inches. The distance between the parallel chords is about ^ of an inch, and theperpendicular of the arcs are each of an inch, giving about 2’2 inches for the total distancebetween the centres of the arcs.

There are two methods of stretching the spring, by pressing it in the direction of theperpendicular of the two arcs which form it, or by drawing it with the two rings at rightangles to that perpendicular : these two limits of tension are indicated by two scalesdrawn on the same limb, called scales of pressure and tension : the first gives the pressureof weight from zero to 264 pounds avoirdupois. The greatest pressure brings the centres

Fig. 1632.

SHAFT OF A MINE.