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 weio-ht moved upon rollers proceeds at twice the rate of the roller, for if 

 C.Jlg. 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 

 instant 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 as far from D as 

 C is, describes E e twice as great a distance as 

 C c ; fresh points are now brought to the summit 

 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 contact 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 continually supplied in front — an incon- 

 venience 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 statue 

 of Peter the Great at St. Petersburgh. 



Fiff. 20. Fiff. 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 



