ON DRAUGHT. 529 



If we wish to destroy the impetus of a body moving with violence, we receive 

 it with a yielding resistance ; the action of catching a cricket-ball exemplifies 

 this perfectly ; and therefore, if the full effect of momentum is wanted, all 

 elasticity in the direction of the movement should be avoided. 



We have entered rather fully into the consideration of this particular point, 

 because the principle is not only applicable to the mode of communicating the 

 immediate action of the moving power, but will be found also of considerable 

 importance when we arrive at the subject of wheel-carriages. 



A consideration of these various points brings us to this conclusion, that the 

 draught ought neither to be constantly uniform nor without remission, nor yet 

 yielding or elastic : sudden shocks or violent changes in the velocity must also 

 evidently be disadvantageous, as tending to distress arid injure the animal. 



Having determined upon the necessary quality of the resistance, we will 

 proceed to examine into the quantity or the degree of resistance or draught, 

 and the speed best adapted to the exertion of the animal. The useful effect of 

 a horse, or the work done, must evidently depend upon three things, viz. the 

 rate at which he is made to travel, the power of traction he can exert, and the 

 number of hours he can continue to work daily at that speed; and where there is 

 no fixed condition which determines any one of these, such as a particular 

 load to be moved, or a certain velocity which it is desirable to attain, or a 

 limited time to perform the work in, then the object must be to search for 

 those proportions of the three by which, at the end of the day, the greatest 

 quantity of work shall have been produced. 



With respect to the first two, viz., the speed and power exerted, it will be 

 obvious, that where a horse travels unloaded, the greatest distance he can go in 

 any given time for several days in succession without injurious fatigue, is the 

 limit of his velocity : on the other hand, the load may be so great, that he can 

 scarcely put it in motion this is the limit of his power : in both cases, the 

 useful effect is nothing. But between these limits of velocity and power, there 

 is a proportion which affords the maximum quantity of effect, and which, there- 

 fore, must be the most advantageous for the application of horse-power. 



It has been asserted by theorists, and the theory appears to be supported by 

 experience, that the velocity corresponding to this maximum, or that at which 

 a horse working continually a certain number of hours per day will do the most 

 work, is equal to half the extreme or limit of velocity of the same horse working 

 the same number of hours unloaded ; and that the force of traction correspond- 

 ing to this speed, is equal to half the limit of his power. For instance, if six 

 hours be the length of a day's work decided upon, and if a horse working that 

 time can go six miles per hour unloaded, and therefore producing no useful 

 effect, and supposing the limit of power of the same horse be equal to 250 Ibs., 

 it is found that he will do the most work in the same number of hours when 

 drawing a load at the rate of half six, or three miles per hour ; and half of 250, 

 or 125 Ibs. will be the strain corresponding to this speed. Our next step, then, 

 must be to find these limits : now, the limit of velocity depends upon the length 

 of time during which the speed is kept up; we subjoin therefore a Table deduced 

 from experiments, and which represents the proportion of the duration of 

 labour and maximum velocity of the average of horses accustomed to their 

 respective velocities. 



Hours. 

 Duration of labour .... 12345 678 10 



This within the range here given may be considered as very nearly the law of 

 decrease of speed by increased duration of labour ; and at the first glance we 

 see the great advantage of reducing the speed and prolonging the exertion 

 There are, however, many causes to limit the duration of a day's work of a 



M M 



