+------+----------+------------+---------+---------------+ | | Diameter | Diameter of| | No. of 3-inch | | H.-P.| of Air | Steam | Stroke. | Rock Drills | | | Cylinder.| Cylinder. | | driven. | +------+----------+------------+---------+---------------+ | | Inches. | Inches. | Inches.| | | 28 | 12 | 12 | 22 | 7 | | 32 | 13 | 13 | 24 | 8 | | 40 | 14½ | 14½ | 28 | 10 | | 60 | 17½ | 17½ | 36 | 15 | | 80 | 20 | 20 | 36 | 22 | | 120 | 22 | 24 | 60 | 30 | +------+----------+------------+---------+---------------+ DURABILITY OF ROPES. The average duration of flat, wire ropes is usually taken at one year, and that of round ropes at half a year. DIAMOND DRILLING. This drill is applicable to sinking a borehole for prospecting for minerals or water, shafts, &c., or blasting under water. It consists of a circular row of “carbonados,” a species of diamond, set in a circular steel ring. This is attached to a hollow steel tube which is kept rotating at about 250 revolutions per minute, pressed forward by a force varying from 400 to 800 lb. according to the nature of the rock. Water is supplied through the tube which washes out the _débris_ and cools the diamonds. Granite and the hardest limestones are penetrated at the rate of 2 to 3 inches per minute, sandstones 4 inches, quartz 1 inch. The diamond drill is not effective in soft strata such as clay, sand, and alluvial deposits. Boreholes have been made at the following rates: _In Ironstone formation_: A depth of 902 feet in 54 working days. 641 ” 48 ” 434 ” 54 ” 640 ” 60 ” Or an average of 12 feet a day. _In Coal measures_: A depth of 1008 feet in 146 days 802 ” 168 ” 700 ” 48 ” 558 ” 42 ” Or an average of 7½ feet a day. TIMBER. =To find Solidity of Round Timber.= _When all dimensions are in feet_: Length × (¼ mean girth)² = cubic feet. _When length in feet_, _girth in inches_: Length × (¼ mean girth) ÷ 144 = cubic feet. _When all dimensions are in inches_: Length × (¼ mean girth)² ÷ 1728 = cubic feet. =To find Solidity of Square Timber.= _When all dimensions are in feet_: Length × breadth × depth = cubic feet. _When one dimension is in inches_: Length × breadth × depth ÷ 12 = cubic feet. _When two dimensions are in inches_: Length × breadth × depth ÷ 144 = cubic feet. For board measure, depth always equal 1 inch. To find surface in square feet, proceed as per rules for solidity of square timber. LAYING OUT OF AREAS.