DRILL. 465 DRILL. which slides on the rod. Between this casting and the collars on the rod are coiled springs through which the power is transmitted for movenient of the arm in each direction. At the heel of the rod is a ritled section or rotchet-rod working in a ritled nnt, hy which the rotary •movement is iiivcu to the drill-bit. rotations per minute. Some of the water, when freed, goes through the hollow bit, keeping it cool and clearing the hole of debris. . watcr- ])rcssure of from 300 to 450 pounds ])er square inch is required to operate these drills. Rock drills are extensively used in quarrying, tunnel- ing, mining, and rock excavation of all kinds, Drill Chuck. Sectional Ptar Fig. 2. ELECTRIC DRILL. FlK. 3. IilAMOND DRILL BIT. Rotary drills differ from percussion drills in the fact that the drill makes its way into the rock by a boring action produced by the axial rotation of the drill-rod. Tlie most common form of rotary drill is the diamond drill. This machine consists of a hollow cjlindrical bit hav- ing a cutting edge of diamonds (Fig. 3), which is revolved at the rate of from •200 to 400 revolutions per minute, by suitable machinery operated by steam or com- pressed air. The diamonds are set in the cutting edge of the bit. so as to project outward from its annular face and also slightly inside and outside its cylindrical sides. When the drill-rod with the bit attached is rotated and thrust forward, the bit cuts an annular hole in the rock, the drillings being removed from the hole by a con- stant stream of water, which is forced down the hollow drill-rod and emerges carrying the dCbris with it up through the narrow space between the outside of the drill-rod and the walls of the hole. Owing to the hollow bit, the nucleus of the hole bored is taken out as a solid core. This possi- bility of the diamond drill makes it a valuable tool for prospecting for minerals, or for deter- mining the character of the material which will be met in sinking deep foundations or in driving tunnels. An- other form of rotary drill which is employed in Eu- rope for tunnel work is the Brandt . hydraulic drill. The cutting edge of this drill consists of hardened steel teeth (Fig. 4). The bit is pressed against the rock by hydraulic pressure, and usuallv makes from seven to eight FlO. 4. BBiSDT ROTARY DRILL BIT. and for prospecting for minerals. For a coneisa history of the development of rock drills, consult Drinker, TunneUmi, Exjilosive Compoundii, and Hock Urills (New York, 1901). DRILL (Dan. drille, bore, Dutch drillen, to bore). In agi-iculture, an implement employed for planting seeds or roots in regular rows. The act of 'drilling' is distinguished from hand or broadcast sowing. In gardening, drilling has been practiced everywhere from time immemorial, but its extension to field culture is of compara- tively recent date. Jethro Tull invented a drill- ing machine in the earl.y part of the eighteenth century, and did much to show its merits in the culture of grain and root crops. Since his time the use of this implement has become very gen- eral. A groat variety of drills are now in use. Some of them are adapted to the distribution of fertilizer along with the seed. One great recom- mendation of drilling is that by means of it a considerable saving of seed is effected, and that especially with close planted crops, such as wheat, the seed is more regularly sown and better covered: a further advantage is that with crops which are cultivated between the drills it en- ables the farmer more readily to clean the land, both by the hand and by the horse hoc. Reten- tion and absori)tion of the moisture is facilitated by keeping the soil stirred and pulverized, which can only be properly done where the crops have been sown in drills. See Implements, Agbicul- TUR.I.. DRILL. A general title for the various sys- tems of exercises and methods of instruction by which organized bodies of men are made uni- formly eriicient and elTcctive for the purposes of the organization of which they are a part. Specifically it has direct reference to the diller- ent details of training by which soldiers and sailors are organized and qualified for their duties. The various European manuals of drill
