554 OX DRAUGHT. 



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 

 Fig. 25. mere inspection of Jig. 25 is suffi- 



cient to show that the velocity of 

 the centre, C, will be to that of the 

 body resting on the point B, as CD 

 to B D, so that if the ends of the 

 rollers are twice the size of the 

 intermediate part, C D Avill 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 re- 

 sistance IS somewhat diminished by having larger rollers in contact ^dth 

 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 supposi- 

 tion is quite gratuitous, as we have no record of any such contrivance 

 having existed before wheels Avere 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, ninety 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 near 

 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-Avheels 

 (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 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 inconsiderable part 

 of the whole i-esistance, or draught of the carriage. 



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

 principle appears to us so simple as to have been necessarily the result of 

 pure invention, almost of inspiration ; while, at the same time, it is so ex- 

 ceedingly effective and perfect, as hardly to admit of improvement. 



The great antiquity of wheeled carriages or chariots precludes all hopes 

 of discovering their origin. About fifteen hundred years before the Chris- 

 tian era they appear to have been in common use amongst the Egyptians 



