Ground augers, Figs. 50, 51, and 56, are similar in action to those used for boring wood, but differ in shape and construction. The common earth auger, Fig. 50, is 3 feet in length, having the lower two-thirds cylindrical. The bottom is partially closed by the lips, and there is an opening a little up one side for the admission of soft or bruised material. Augers are only used for penetrating soft rock, clay, and sand; and their shape is varied to suit the nature of the strata traversed, being open and cylindrical for clays having a certain degree of cohesion, conical, and sometimes closed, in quicksands. Augers are sometimes made as long as 10 feet, and are then very effective if the strata is soft enough to permit of their use. The shell is made from 3 feet to 3-1/2 feet in length, of nearly the same shape as the common auger, sometimes closed to the bottom, Fig. 56, or with an auger nose, Fig. 51; in either case there is a clack or valve placed inside for the purpose of retaining borings of a soft nature or preventing them from being washed out in a wet hole. Fig. 59 shows a wad-hook for withdrawing stones, and Fig. 58 a worm-auger. The Crow's Foot, Fig. 55, is used when the boring rods have broken in the bore-hole, for the purpose of extracting that portion remaining in the hole; it is the same length, and at the foot the same breadth as the chisels. When the rods have broken, the part above the fracture is drawn out of the bore-hole and the crow's foot screwed on in place of the broken piece; when this is lowered down upon the broken rod, by careful twisting the toe is caused to grip the broken piece with sufficient force to allow the portion below the fracture to be drawn out of the bore-hole. A rough expedient is to fasten a metal ring to a rope and lower it over the broken rod, when the rod cants the ring, and thus gives it a considerable grip; this is often very successful. Fig. 57 is a worm used for the same purpose. A bell-box, Fig. 60, is frequently employed for drawing broken rods; it has two palls fixed at the top of the box, which rise and permit the end of the rod to pass when the box is lowered, but upon raising it the palls fall and grip the rod firmly. A spiral angular worm, similar to Fig. 57, is also applied for withdrawing tubes. [Illustration: Withdrawing Tools. Figs. 57-59.] [Illustration: Bell-box. Fig. 60.] Of these withdrawing tools the crow is the safest and best, as it may be used without that intelligent supervision and care absolutely necessary with the worms and wad-hooks, or the bell-box.