ox DRAUGHT. 



553 



operations tlien commenced. The railway consisted of two lines of timber 

 a a a a, (figs. 20, 21, 22,) furnished with hard metal grooves ; snndar and 



Fiy. 22. 



corresponding metal grooves were fixed to the under side of the sledge 

 and between these grooves were placed the rollers, which were spheres ot 

 hard brass, about "six inches diameter. The xmpossibdi y of conhnmg 

 cvHndrical rollers to a perfectly parallel direction, and without which the 

 Sc on 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 npon which they can bear, and the consequent danger of 

 crushino- or at least flattening that surface, is a serious objection to 

 spheresT once placed npon the rollers, it was dra^vn by means ot capstans. 

 The resistance does not appear to have been great, considering the enor- 

 mous weight, since sixty men at the capstans with treble purchase block. 



moved it with ease. , -, . , , 



The transport of this enormous rock tmder such disadvantageous cir- 

 cumstances of country, over a distance of four miles, and its subsequent 

 pas ao^ of thirteen miles by water in a vast caisson or vessel constructed 

 fortfe purpose, wasaworksuiTassinganythingattempted by the ancients 



and indeed, in'modern times the only thing which can be compared to i 

 is the drao-J-inga ship of the line up a shp ; the weight is m this case 

 nearly l^e^^am^e 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 

 prepaml and well greased; a frame of wood, technically called a cradle, 

 Tfixed under the vessel, it is floated on.to the mclmed 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 

 siu^ace being well covered with 

 gTsase to lessen the friction. 



We have stated that there Avas a 

 particular construction of roller 

 which might be considered, as re- 

 gards its form merely, an inter- 

 mediate step between the roller and 

 the wheel. It consists of a roller 

 with the diameter of the extremities 

 increased, as in fig. 23; the only 

 advantao:e of this roller is that the 

 body rests upon the small part of 

 the roller, see fig. 24, and when put 



Fig. 23. 



Fig. 24. 



