In veins of coal of a thickness from twenty-eight to thirty inches, such a machine will cut about one hundred tons of coal in a day. The cost of production with such machines has been estimated at about sixty-three cents a ton, as against ninety cents as the cost of pick mining in rooms,--a saving of about twenty-seven cents a ton. Since it is estimated that for a cost of $10,000 an electrical equipment can be installed capable of working four such machines besides affording power for lighting, pumping, ventilation of the mine, etc., thus saving something like $100 a day for the operator, the great popularity of these machines is readily understood. After such a machine has been placed in position, a cut some four feet wide, four or five inches high, and six feet deep can be made in five minutes, with the expenditure of very little energy on the part of the workmen. One of the largest cuttings ever recorded by one of these machines is 1,700 square feet in nine and one-half hours, although this may have been exceeded and not recorded. Among the several advantages claimed for the chain machine over the older pick machines is the small amount of slack coal produced, and the absence of the racking vibrations that exhaust the workmen, and, like the air drills, sometimes cause serious diseases. On the other hand the advocates of the pick machines point out that they can be used in mines too narrow for the introduction of chain machines. They show also that there is a constant element of danger from motor-driven machines in mines where the quantity of gas present makes it necessary to use safety lamps, on account of the sparking of the machines which may produce explosions. Both these claims are valid, but apply only to special cases, or to certain mines, and do not affect the general popularity of the chain machines. There are several different types of chain cutting machines, such as "long-wall machines," and "shearing machines," but these need not be considered in detail here. The general principle upon which they work is the same as the ordinary chain machine, the difference being in the method of applying it for use in special situations. ELECTRIC LIGHTING OF MINES For many obvious reasons the ideal light for mining purposes is one in which the danger from the open flame is avoided, particularly in well-ventilated mines, or mines under careful supervision, where the danger from inflammable gases is slight. The incandescent electric light, therefore, has become practically indispensable in modern mining operations. For certain purposes and in certain locations where an intense light is desirable and where there is no danger from combustible gases, arc lights are used to a limited extent. But there is constant danger from the open flame in using such lights, and also from the connecting wires leading to them. Furthermore, such intense light is not usually necessary in the narrow passages of the mine.