430 



ON DRAUGHT. 



and the operations then commenced. The railway consisted of two lines 

 of timber a a a a, (figs. 20, 21, 22,) furnished with hard metal grooves; 



Fig. 22. 



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 diameter. The impossibility of con- 

 fining cylindrical rollers to a perfectly parallel direction, and without which 

 the friction would have been considerable, rendered the adoption of sphe- 

 rical 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 capstans. 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 work surpassing anything of the sort attempted by the 

 ancients, and, indeed, in modern times the only thing which can be com- 

 pared 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 inchned 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 



Fig. 23. 



between the roller and the wheel. 

 It consists of a roller with the 

 diameter of the extremities in- 

 creased as in Jig 23 ; the only 

 advantage of this roller is that 

 the body rests upon the small 

 part of the roller, see fig. 24, and 



