•^^8 ON DRAUGHT. 



smooth, level, and plane ; we need hardly say that such can never be the 

 case m a road. The friction, however, remains, practically speakin?-, the 

 same, and the laws which govern the amount and the effects of it remain 

 unaltered ; and we have only to ascertain what is the additional resistance 

 arising from other sources, to obtain the whole draught of the carriao-e. 

 We have already stated, when pointing out the difference between tlie 

 roller and the wheel, that the movement of the latter was attended with 

 two sources of resistance, viz. friction at the centre, which we have con- 

 sidered, and another, which is common both to the wheel and the roller, 

 arising from impediments in the road, or the yielding of the materials. 

 _ The laws which affect the amount of this latter are, of course, the same 

 111 a wheel as in a roller. 



We have found that the power required to overcome it is inversely as 

 the square root of the diameter ; therefore, by increasing the diameter of 

 the wheel, the effect of friction, which is inversely as the diameter dimi- 

 nishes much more rapidly than that caused by impediments in the 'roads ; 

 and on ordinary roads, with common carts, the amount of the latter is 

 about three times as great as that of the former, and when the roads are at 

 all injured by weather or by neglect, or if they are naturally heavy or 

 sandy, it bears a much greater proportion. A light four-wheeled cart 

 weighing, with its load, 1,000 lbs., was repeatedly dra^vn upon different 

 ?°AnA n^ ^^T^^' "^^^ experiment was not made with a load of exactly 

 1,000 lbs., but the proportions of the results are calculated to this standard 

 The public are indebted to Mr. Bevan for these as well as for a great 

 number of other highly useful and practical experiments upon the effects 

 of power m various cases. The average of a number of experiments o-ave 

 the following results : — ^ 



Description of Road. Force of Traction required 



n^ ■■,■,,, ., ff> niove the Carriage. 



1 urnpike-road— hard, dry .... 30^ lbs. 



Ditto dirty \ . . ■ . 39^ 



Hard, compact loam . . . . .53 



Ordinary by-road . . . . 106 



Turnpike-road — new gravelled . . . 143 



Loose, sandy road .... 204 



The friction at the axles, which were of wood, was, of com-se, nearly- 

 constant, and probably absorbed at least -^\ of the weight, or 12^ lbs. of the 

 force of traction, lea\ang, therefore, for the resistance caused by the road 

 in the different cases, as under — 



Descriptio-.i cf liitvi ^°''°^ °^ Traction required to move the Carriage, 

 „ ., , , , independent of the Friction at the Axles. 



1 urnpike-road — hard, ar^-, about . . 18 lbs. 



Ditto dirty . . . . 2Gt- 



Ditto new gravelled . . . isqI 



Loose, sandy road . . . 191^ 



So that in the last ease, one by no means of rare occurrence in many parts 

 of the country, the portion of draught immediately caused by the state of 

 the road was ten times as great as on a good turnpike-road, and about 

 lifteen times as great as that which arose from friction at the axles. It 

 would be hopeless to attempt to remedy this by increasing the size of the 

 wheel ; the experiment was made with wheels of the ordinary size. To 

 double their diameter would evidently be attended, in practice, with 

 insurmountable difficulties ; and yet, even if this were effected, it would 

 barely reduce the total amount of the draught by one-fom-th ; but the form 

 of the wheel may materially influence the state of the road ; we shall, 

 therefore, proceed to consider the various forms employed. 



Some years ago, when the principal turnpike roads of the kingdom were 

 at many parts, at particular seasons of the voar, in httle bette? condition 



