450 



ON DRAUGHT. 



anJ the optsrations then commenced. The railways consisted of two lines 

 of timbei a a a a, (figs. 20, 21, 22,) furnislied with liard metal grooves* 



similar and corresponding metal grooves were fixed to the under side of the 

 sledge, and between these grooves were placed the rollers, which were 

 spheres of" hard brass, about six inches in diameter. The impossibility of 

 confinmg cylindrical rollers to a perfectly parrallel direction, and without 

 which the friction would have been considerable, rendered the adoption of 

 spherical rollers or balls running in a groove a matter of necessity, as 

 otherwise 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 cap- 

 stans. The resistance does not appear to have been great, considering the 

 enormous weight, since sixty men at the capstans with treble purchase 

 blocks moved it with ease. 



The transport of this enormous rock under such disadvantageous cir- 

 cumstances 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 M'ork surpassing any thing of the sort 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 sur- 

 face 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 roller and the wheel. 



Fig. 23. 



It consists of a roller with the 

 diameter of the extremities increased 

 as in jig. 23 ; the only advantage 

 of this roller is that the body rests 

 upon the small part of the roller, 

 see Jig. 24, and when put in motion, 



