acquired by a few days' labour: yet when this is employed for the purpose alluded to, effects are frequently produced which could not, even with the best tools, be accomplished by other means in less than many months.
The dimensions of one of the blocks of limestone extracted from the quarries worked for the formation of the breakwater at Plymouth, were 2612 ft. long, 13 ft. wide, and 16 ft. deep. This mass, containing above 4800 cubic feet, and weighing about 400 tons, was blasted three times. Two charges of 50lbs. each were successively exploded in a hole 13 feet deep, the bore being 3 inches at top and 212 inches at bottom: 100lbs. of powder were afterwards exploded in the rent formed by those operations. Each pound of gunpowder separated from the rock two tons of matter, or nearly 4500 times its own weight. The expense of the powder was 6l., or nearly 712d. per lb.: the boring occupied two men during a day and a half, and cost about 9s.; and the value of the produce was, at that time, about 45l.
(7.) The simple contrivance of tin tubes for speaking through, communicating between different apartments, by which the directions of the superintendant are instantly conveyed to the remotest parts of an establishment, produces a considerable economy of time. It is employed in the shops and manufactories in London, and might with advantage be used in domestic establishments, particularly in large houses, in conveying orders from the nursery to the kitchen, or from the house to the stable. Its convenience arises not merely from saving the servant or workman useless journeys to receive directions, but from
