In drilling for oil the taking of core samples of the beds penetrated is becoming a common practice. With the rotary drill the rock cuttings are ordinarily swept out by a vigorous forced circulation of sludge in the hole, but it has been found possible in many cases to modify the procedure in such a way that a core of moderate length can be obtained. The usual bit is dispensed with and a length of ordinary drill pipe with teeth cut at the end is substituted. This is rotated at the bottom of the hole, and under favorable circumstances it cuts through the rock and furnishes a cylindrical core. The presence of the core prevents the circulation of water to the bottom of the hole; indeed, a wooden plug is sometimes inserted in the top of the core barrel to guarantee this condition. With the free circulation of water . . .