The drilling-jars (Fig. 74) are generally not used until the hole is 150 ft. deep or more. They resemble two great links of a chain with about 16-in. stroke for ordinary drilling. When the tools be come fast from cavings or any other cause, the jars, by lowering the temper-screw, are slacked sufficiently to deliver a sharp upward blow, eliminating the strain on the drilling-line, which would occur if pulling were resorted to. In ordinary drilling, the jars are not brought into action, but remain extended to their full stroke. The stem (Fig. 76) with 3 by 4-7 joints is usually 4Y2 in. by 28 ft. long, and a complete string of tools of this size weighs about 4000 pounds.
In districts where the formation is slate, limestone or sandstone, it is usual to dress the cutting-edge of the bit more or less to a chisel point in order to make faster headway in the hard rock, while in soft formations of clay, shale or sand, the centre of the bit is cut out, making a concave surface with the outer edges from 1 to 3 in. longer than the centre. In either case, all four corners are drawn out to gauge and the cutting-edges properly rounded off to conform to the size of the casing used. In California, the shank of a drilling bit should be smaller than the cutting edge by 1 or 2 in., thus affording an offset by which a larger hole can be cut than with a straight bit. In soft formations, a chisel-bottom bit will dig faster than the drillings can be mixed with the water, making it necessary to re-drill the debris in order entirely to re move it from the hole, while a concave bit is totally unsuited to hard formation, as a sharp, cutting edge is desired. Bits are dressed, therefore, to suit the forma tion. Large water-courses are provided in the Cali fornia style of bit (Fig. 77), which mixes the water more freely with the drillings; some operators prefer the 'Mother Hubbard' pattern (Fig. 78), as the square shoulders help in mixing the mud, and when this bit unscrews or is lost, usually stands straighter in the hole than those with a rounded shoulder, making its withdrawal much easier. The occasion often arises where the use of the under-reamer is impossible for reaming a hard formation or shell, in which case a bit can be dressed `sidehill,' that is, with one lug or cutting edge drawn out 1 to 2 in. larger than gauge, while the other edge is beaten in somewhat, making a one-sided tool which cuts a larger hole than would the ordinary bit. Sidehill bits are often used when drilling in stove
pipe casing where the under-reamer cannot be used.
The under-reamer (Figs. 79, 80, 81, 82) is a specially designed tool which, as its name implies, is used to ream or enlarge the hole below the casing and is employed constantly in wells where it is desired to carry the strings of pipe for long distances. The Cali fornia tinder-reamers are reliable in construction and action ; they have two lugs. or cutters, which, when fully expanded, will cut a larger hole than would the casing shoe, giving the casing ample room between the walls. A 10 in, under-reamer, for instance, will cut a 13-in. hole, while the 10-in. shoe is 12 in. diameter, leaving a space of 1% in. These cutters are held in place by a power ful spring and can be pulled down to a smaller diameter than the inside of the pipe. When its use is required, the bit is removed and the under-reamer attached to the stem, the cutters are pulled to gether on the derrick floor by the driller, and the string of tools lowered in the well. Upon emerging from the shoe, the spring ex pands the cutters back to a shoulder on the body of the under reamer. Then they are ready for work. Upon being withdrawn, the cutters strike the shoe and are pulled together, after which the tools can be raised to the surface. The wrenches. (Fig. 84) for set ting up the joints are massive, weighing from 250 to 450 lbs. each, and are usually counterbalanced by weights suspended outside of the derrick. The swivel wrench (Fig. 85), which hangs from the traveling hoist running on the crane, is used for holding the tools in place when being screwed together.
For removing drillings from the hole different designs of hailers are used, the working principle the same in .all, that is, a valve is placed at the bottom of a smaller size of pipe than the casing being drilled and a bail is riveted at the top. The valve opens when it strikes the mud or water and closes when the bailer is lifted from the well. In the flat-bottom bailer, a hinged valve upon a flat seat is used, while in the dart-bailer (Fig. 87), a with a dart for a guide and a seat answers the purpose. In the Morahan (Fig. 88), or other special forms, suction is provided by means of a long plunger with a valve at the bottom to remove sand or broken particles of iron from the hole.