6. The same stone was now placed upon rollers of three inches diameter, when it required to put it in motion along the floor of the quarry 34 lbs 7. To drag it by these rollers over a wooden floor 28 lbs 8. When the stone was mounted on a wooden platform, and the same rollers placed between that and a plank floor, it required 22 lbs From this experiment it results, that the force necessary to move a stone along Part of its weight The roughly chiselled floor of its quarry is nearly 2/3 Along a wooden floor 3/5 By wood upon wood 5/9 If the wooden surfaces are soaped 1/6 With rollers on the floor of the quarry 1/32 On rollers on wood 1/40 On rollers between wood 1/50 At each increase of knowledge, as well as on the contrivance of every new tool, human labour becomes abridged. The man who contrived rollers, invented a tool by which his power was quintupled. The workman who first suggested the employment of soap or grease, was immediately enabled to move, without exerting a greater effort, more than three times the weight he could before.(5*) 6. The economy of human time is the next advantage of machinery in manufactures. So extensive and important is this effect, that we might, if we were inclined to generalize, embrace almost all the advantages under this single head: but the elucidation of principles of less extent will contribute more readily to a knowledge of the subject; and, as numerous examples will be presented to the reader in the ensuing pages, we shall restrict our illustrations upon this point. As an example of the economy of time, the use of gunpowder in blasting rocks may be noticed. Several pounds of powder may be purchased for a sum 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 26 1/2 ft long, 13 ft wide, and 16 ft deep. This mass, containing above 4,800 cubic feet, and weighing about 400 tons, was blasted three times. Two charges of 50 lbs each were successively exploded in a hole 13 feet deep, the bore being 3 inches at top and 2 1/2 inches at bottom: 100 lbs 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 4,500 times its own weight. The expense of the powder was L 6, or nearly 7 1/2d. 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 L 45.