ON DRAUGHT. 543 



freight moved; therefore, as the weight is forced forward, it moves at twice the 

 rate of the roller, it will gain upon the rollers, and others must be continually 

 supplied in front — an inconvenience much felt in practice. 



This confines the use of the roller to cases where the distance is very short 

 or where the weight conveyed is exceedingly great, and reduction in the resist! 

 ance of more importance than the inconvenience alluded to. 



The most remarkable instance of the application of rollers is the transport 

 of the rock which now serves as the pedestal of the equestrian statue of Peter 

 the Great at St. Petersburgh. 



Fig. 20. 



Fig. 21. 



This rock, a single block of granite, was discovered in the centre of a bog, 

 four miles from the waterside; it weighed, after being cut into a convenient 

 shape, 1217 tons. Notwithstanding its enormous weight it was raised and 

 turned upon its side, and placed upon a frame. A road was made across the bog, 

 and a timber railway laid down ; the whole was then left till the depth of 

 winter, when the boggy ground was frozen 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 ; 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 im- 

 possibility of confining cylindrical rollers to a perfectly parallel 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 other- 



