ON DRAUGHT. 



431 



i'V- 24. 



will not gain so rapidly on the roll 

 ers ; or, in other words, the roller 

 will move with more than half thp 

 velocity of the body. A mere in 

 spectiou of fig. 25, is sufFicien. 

 to show that the velocity of the 

 centre, C, will be to that of the 

 body resting on the paint B, as 

 C D to B D, so that if the ends of 

 the rollers are twice the size of 

 the intermediate part, C D will be 

 equal to two-thirds of B D, and 

 the roller will move at two-thirds 

 of the rate of the body ; a less 

 number of rollers are therefore 

 required, and the resistance is 

 somewhat diminished by having 

 larger rollers in contact with the 

 ground. 



In using a roller of this sort, the 

 idea may have struck the workman, or it may have occurred accidentally, 

 to confine the spindle of the roller, and compel it to move with the body; 

 and thus a clumsy pair of wheels, fixed to a spindle, would have resulted 

 from his experiment. Such a supposition is quite gratuitous, as we have 

 no record of any such contrivance having existed before wheels were 

 made ; indeed it is inferior both to the roller and the wheel : the only argu- 

 ment in favour of such a theory is, that rollers of this sort have been em- 

 ployed in comparatively modern times. 



At Rome, in 1588, an obelisk, 90 feet high, of a single block of stone, 

 weighing upwards of 160 tons, and which had originally been brought 

 from Egypt, was removed from one square, in which it stood, to another in 

 the Vatican, and there again erected in the spot where it now is. 



In dragging this through the streets of Rome, it was fixed in a strong 

 frame of wood, which rested upon a smaller frame, which were furnished 

 each with a pair of rollers, or spindles, of the form above referred to; they 

 were turned by capstan bars: indeed, they cannot be better described than 

 by stating that they resembled exactly the naves of a pair of cart wheels 

 (all the spokes being removed), and fixed to a wooden axle. If a heavy 

 waggon lay upon a pair of these, we can conceive that by putting bars into 

 the mortices of the nave, we could force them round, and thus advance the 

 waggon ; but the resistance would evidently be greater tlian if either roll- 

 ers or wheels were employed. 



All the difficulties incidental to the use of the roller appear to be sur- 

 mounted, and all objections met, by the contrivance of the wheel. 



The wheel being attached to the load, or to the carriage which contains 

 it, moves with it, is pan of the machine, and consequently as we require 

 only the number of wheels immediately necessary for the support of the 

 load, we can afford to construct them of those dimensions and materials 

 best suited to the purpose. By increasing their diameter, we are enabled 

 to surmount impediments with much greater facility, as we have shown in 

 the case of the roller ; and although there is a resistance arising from 

 friction at the axle, which does not exist in the roller, yet this may be so 

 reduced, by increasing the diameter of the wheel, as to form an incunsider- 

 able part of the whole resistance, or draught of the carriage. 



Of the first introduction of the wheel we have no record whatever 



