ON DRAUGHT. 



429 



great weight may be moved with comparatively small power ; but, on the 

 other hand, there is a serious practical inconvenience attending the use of 

 a roller, which prevents its adoption except in very particular cases. 



A weight moved upon rollers proceeds at twice the rate of the roller, for 

 i^C.fig. 19, be the centre of the roller, D the point 

 of contact with the ground, and E that with the 

 weight to be moved, and W the weight, if this 

 weight be put in motion, the point D is for an in- 

 stant stationary, since it is in close contact with the 

 ground. The diameter E C D moves, therefore, 

 round the point D as a centre, and, consequently, 

 E being twice a« far from D as C is, describes E e 

 twice as great a distance as C c; fresh points are 

 now brought to the sunimit and in contact with the 

 ground, and again the latter is stationary, while the 

 former moves twice the distance which the point C 

 does. The summit, therefore, or that point which 

 is in immediate contact with the weight, always moves with twice the 

 velocity of the centre of the roller; but the velocity of the centre is, of 

 course, that of the roller, and the velocity of the point E, which is in con- 

 tact with, and is moved by, the weight, is the same as that of the weight 

 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 continu- 

 ally 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 resistance of more importance than the inconvenience alluded to. 



The most remarkable instance of the application of rollers is the trans- 

 port of the rock which now serves as the pedestal of the equestrian statute 

 of Peter the Great at St. Petersburgh. 



Fig. 20. 



Fig. 21. 



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

 l)og, four miles from the waterside ; it weighed, after being cut into a 

 cTTivenient 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 Irozen 



