528 ON DRAUGHT. 



For it is a well-known fact, that, however powerful may be the muscles of a 

 limb, they must not be kept constantly on the stretch. Thus we feel even more 

 fatigue by standing than by walking, because one particular set of muscles 

 is then kept constantly exerted. It is evident, therefore, that the resistance or 

 draught must not be perfectly constant, but should afford frequent opportunities 

 of relaxing the efforts. Neither must it be a yielding resistance, as in that case 

 the animal could not make any great exertion ; for if he applied too much power, 

 he would be liable to fall forward ; and should he at any time fall short of the 

 necessary exertion, he would be drawn back by the strain, and it would require 

 a considerable effort to restore the motion. 



If a horse be made to drag a rope passing over a pully and descending into 

 a well with a certain weight, say of 2001bs. attached to it, it is obvious that he 

 could not make an effort greater than 200lbs. without instantly considerably 

 increasing his velocity, which would be a waste of power ; nor must he for an 

 instant relax his efforts, or fall below that mark, for he would then be unable 

 even to resist the pull, and would be overcome by the weight. Such an extreme 

 case as this, of course, is not likely to occur often in practice, but the disadvan- 

 tage of the principle is obvious. 



An arrangement of this sort is, indeed, sometimes made use of, for raising 

 the earth from excavations, or the materials of a building ; but the exertion is 

 continued only for a few seconds, or for a distance of not more than ten or 

 twenty yards : if prolonged, the inconvenience would be seriously felt, as it is, 

 to a certain degree, in towing canal boats ; the length and curve of the rope give 

 an elasticity to the strain, and the necessity of keeping the rope out of the 

 water, or from dragging along the towing-path, compels the animal to keep up 

 a constant, unremitted pull, and that, too, in an oblique direction, so as to 

 throw him into an unfavourable position. We accordingly find that, in 

 these circumstances, the average work of a horse is equivalent only to about 

 four-fifths of that given by Smeaton, Desaguilliers, and others, who estimated 

 the power of the horse from the work done in a horse-mill, where the resistance 

 is inelastic, and all circumstances favourable, with the exception of the circular 

 path. 



The disadvantage of this kind of resistance is well known to carmen, though 

 of course without consideration of the reason. A horse is said to pull better 

 when he is close to his work, that is to say, when he is attached at once to the 

 body to be moved, because every exertion he makes is then communicated at 

 once to the mass; but the leader of a team, unless he keeps the traces con- 

 stantly on the stretch, may frequently waste a powerful effort without 

 producing much effect upon the carriage. 



Another inconvenience resulting from harnessing horses in a team, or one 

 before the other, is, that the leader, by tightening the traces, is continually 

 relieving the strain from the body horse, and reciprocally the body horse from 

 the leader ; so that these horses labour under all the disadvantages of a long, 

 elastic, and constantly yielding connexion with the load, which is not only 

 fatiguing to them, but, in cases where the resistance is variable, prevents the 

 full and united effect of their exertions being properly communicated to the 

 carriage. For, if a slight obstacle, as a rut or stone in a road, checks the pro- 

 gress of the vehicle, the shaft-horse can immediately throw his whole weight 

 into the collar, and the united effect of his strength and impetus is conveyed 

 unimpaired to the vehicle, and forces it over the obstacle ; but if any elasticity 

 is interposed between the power and the resistance, as in the case of the traces 

 of the leader of a team, the whole, or the greater part of the effect of impetus is 

 lost, and that force which, if concentrated in one effort, would effect the object, 

 being lengthened into a continued and comparatively feeble pull, is insufficient. 



