544 



ON DRAUGHT. 



Fig. 24. 



wise the small surface upon which they can bear, and the consequent danger of 

 crushing, or at least flattening that surface, is a serious objection to spheres ; 

 once placed upon the rollers, it was drawn by means of capstans. The resist- 

 ance does not appear to have been great, considering the enormous weight, since 

 sixty men at the capstans with treble purchase blocks moved it with case. 



The transport of this enormous rock under such disadvantageous circum- 

 stances of country, over a distance of four miles, and its subsequent passage of 

 thirteen miles by water in a vast cassoon or vessel constructed for the purpose, 

 was a work surpassing anything attempted by the ancients, and, indeed, in 

 modern times the only thing which can be compared to it is the dragging a ship 

 of the line up a slip ; the weight is in this case nearly the same as that of the 

 rock, but the distance traversed is short, and the difficulties to be overcome much 

 less. A plane of inclined timber is prepared and well greased ; a frame of 

 wood, technically called a cradle, is fixed under the vessel, it is floated on 

 to the inclined plane and drawn up by the united efforts of a number of well- 

 manned capstans, with powerful tackle : in this case no rollers are used : 

 it is a sledge, the surface being well covered with grease to lessen the friction. 



We have stated that there was a particular construction of roller which might 

 be considered, as regards its form merely, an intermediate step between the 

 Fig. 23. roller and the wheel. It consists 



of a roller with the diameter of the 

 extremities increased as in fig. 23 ; 

 the only advantage of this roller is 

 that the body rests upon the small 

 part of the roller, see fig. 24, and 

 when put in motion, will not gain 

 so rapidly on the rollers; or in 

 other words, the roller will move 

 with more than half the velocity 

 of the body. A mere inspection 

 of fig. 25, is sufficient to show 

 that the velocity of the centre, C, 

 will be to that of the body resting 

 on the point B, as C D to B D, 

 so that if the ends of the rollers 

 are twice the size of the inter- 

 mediate part, C D will be equal 

 to two-thirds of B D, and the 

 roller will move at two-thirds of 

 the rate of the body ; a less num- 

 ber of rollers are therefore re- 

 quired, and the resistance is 

 somewhat diminished by having 

 larger rollers in contact with the 

 ground. 



In using a roller of this sort, the idea may have struck the workman, or it 

 may have occurred accidentally, to confine the spindle of the roller, and compel 

 it to move with the body ; and thus a clumsy pair of wheels, fixed to a spindle, 

 would have resulted from his experiment. Such a supposition is quite gratui- 

 tous, as we have no record of any such contrivance having existed before wheels 

 were made ; indeed it is inferior both to the roller and the wheel : the only 

 argument in favour of such a theory is, that rollers of this sort have been 

 employed in comparatively modern times. 



At Rome, in 1588, an obelisk, ninety feet high, of a single block of stone, 

 weighing upwards of 160 tons, and which had originally been brought from 



Fig. 25. 



