ON DRAUGHT. 



431 



Fig. 24. when put in motion, " will not 



gain so rapidly on the rollers ; or 

 in other words, the roller will 

 move with more than half the 

 velocity of the body. A mere in- 

 spection of fig. 25, is sufficient 

 to shew that the velocity of the 

 centre, C, will be to that of the 

 body resting on the point B, as 

 ^'3' ^^' 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 argument in favour of such a theory is, that 

 rollers of this sort have been employed 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 fur- 

 nished 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 naves, we could force them round, 

 and thus advance the waggon ; but the resistance would evidently be greater 

 than if either rollers 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 part 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 shewn 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 in- 

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



Of the first introduction of the wheel we h^ve no record whatever* 



