But admitting that it is sometimes desirable for nations and governments to erect monuments which are not of direct utility,--which may have an indirect utility in recording the memory of great exploits, or in producing feelings of reverence or devotion,--it is clearly an advantage that these works, as well as all other works, should be performed in the cheapest manner; that is, that human labour should derive every possible assistance from mechanical aid. We will give an illustration of the differences of the application of mechanical aid in one of the first operations of building, the moving a block of stone. The following statements are the result of actual experiment upon a stone weighing ten hundred and eighty pounds. [Illustration: Portland Quarry.] To drag this stone along the smoothed floor of the quarry required a force equal to seven hundred and fifty-eight pounds. The same stone dragged over a floor of planks required six hundred and fifty-two pounds. The same stone placed on a platform of wood, and dragged over the same floor of planks, required six hundred and six pounds. When the two surfaces of wood were soaped as they slid over each other, the force required to drag the stone was reduced to one hundred and eighty-two pounds. When the same stone was placed upon rollers three inches in diameter, it required, to put it in motion along the floor of the quarry, a force only of thirty-four pounds; and by the same rollers upon a wooden floor, a force only of twenty-eight pounds. Without any mechanical aid, it would require the force of four or five men to set that stone in motion. With the mechanical aid of two surfaces of wood soaped, the same weight might be moved by one man. With the more perfect mechanical aid of rollers, the same weight might be moved by a very little child. From these statements it must be evident that the cost of a block of stone very much depends upon the quantity of labour necessary to move it from the quarry to the place where it is wanted to be used. We have seen that with the simplest mechanical aid labour may be reduced sixty-fold. With more perfect mechanical aid, such as that of water-carriage, the labour may be reduced infinitely lower. Thus, the streets of London are paved with granite from Scotland at a moderate expense.