ON DRAUGHT. 539 



This law of the increase of resistance is modified however by other causes, 

 which have been observed and deeply investigated within the last few years, 

 and which produce such an effect, that with boats of a peculiar form, a diminu- 

 tion of resistance actually occurs at a certain increased velocity, and very high 

 rates of speed, such as even 10 or 12 miles per hour, have been attained. There 

 are also some small sources of resistance, such as the friction of the water, 

 which do not increase in the ratio above named, but at moderate velocities the 

 rule applies, and as yet no means have been discovered, by which, with the 

 present dimensions of canals and their locks, larger quantities and weights can 

 be conveyed at any but very low rates of speed. The draught of an ordinary 

 canal-boat, at the velocity of 2£ miles per hour, is about -gfo of its weight, 

 that is to say, a canal boat, with its load weighing 33 tons, or 73,020 lbs., is 

 moved at the rate mentioned, by a force equivalent to 80 lbs., being -g^ F part of 

 the load. This is found by Mr. Bevan to be the result upon the Grand Junction 

 Canal, and a force of traction of 80 lbs., is here found to be equivalent to a 

 horse power. The average power of an ordinary horse is certainly rather more ; 

 and in the commencement of this paper, we mentioned this as an instance of a 

 small effect being produced, most probably owing to the peculiar application of 

 the power. We believe it to be the case, and think it likely, that if the disad- 

 vantages before alluded to, arising from the mode of applying the power, could 

 be removed, the effect might be raised 100 lbs., or 120 lbs. of traction, and con- 

 sequently the load moved would then be 40 or 60 tons ; this is an increase well 

 worthy of consideration. 



We now come to the consideration of the means of transport employed on 

 land. These are sledges, rollers, and wheel carriages. The order in which 

 they are here mentioned, is probably that in which they were invented or first 

 employed. A sledge is certainly the rudest and most primitive form of vehicle ; 

 the wheeled carriage, and even the placing the load itself upon rollers, is the 

 effect of a much more advanced state of the mechanical arts, and is probably of 

 much later date than the sledge. 



When man first felt the necessity or the desire of transporting any article 

 from one spot to another, he doubtless endeavoured to lift or carry it : if it 

 proved too heavy for him to carry, he would naturally endeavour to drag it. 

 Here frequent experiments would soon show him how much less labour was 

 required to drag a body with a smooth surface in contact with the ground, than 

 when the contrary was the case ; and if the body to be moved did not itself 

 present a smooth surface on any of its sides, but was, on the contrary, rough 

 and angular in all directions, he would naturally be led to interpose between it 

 and the ground some plane surface which should prevent the angles and projec- 

 tions of the body from entering the ground and impeding the progress ; and 

 we may presume that sledges were thus very early brought into use. When 

 attempting to transport still heavier masses, the accidental presence of round 

 stones, or of a piece of timber, may have shown the advantage of interposing 

 rolling bodies, and thus may rollers nave been invented and first brought into use. 



These steps appear natural and likely to have led to these results ; they are at 

 any rate sufficient to account for the first introduction of these two means of 

 facilitating transport, but no steps of this kind appear capable of leading to the 

 beautiful yet simple contrivance of a wheel. 



A roller is by no means an imperfect wheel, as it may at first appear to be ; 

 they have nothing in common but their rotatory or revolving action, but the 

 effect of this motion is totally different in the two. In a roller, friction is 

 avoided altogether by it, in a wheel this friction exists as completely as in a 

 sledge, but the sliding surfaces being at the centre of the wheel, instead of on 

 the ground, are always the same, and being under control, may be kept in that 



