ON DRAUGHT. 415 



and the number of hours he can continue to work daily at that sj-^ed ; and 

 wliere there is no fixed condition which determines any one of tiiose, 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 daj 

 the greatest quantity of work shall have been produced. 



With respect to the first two, viz: the speed and power exerted, it will bf> 

 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 etlect is nothing. But between these limits of velocity 

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

 effect, and which, therefore, must be the most advantageous for the applica- 

 tion of horse-power. 



It has been asserted by theorists, and the theory appears to bo .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 corresponding 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 lbs., it is found that he will do 

 the most work in the same number of hours Avhen drawing a load at the 

 rate of half six, or three miles per hour; and half of 250 or 125 lbs. will 

 be the strain corresponding to this speed Our next step, then, must be to 

 frid these limits: now, the limit of -elocity 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 accus- 

 tomed to their respective velocities. 



Duration of labour . 

 Maxinmm velocity unload- 

 ed in miles per hour. 



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 horse. Tredgold, in his work on Railways, before quoted, 

 says: "The time assigned for the day's work of a horse is usually eight- 

 hours: but it is certain, from experience, that some advantage is gained by 

 shortening the hours of labour; and we have observed, that a horse is least 

 injured by his labour, where his day's work is performed in about six 

 hours; where the same quantity of labour is performed in less than six 

 hours, the over-exertion in time shows itself in stiffened joints, while the 

 wearying efiects of long-continued action become apparent, if the duration 

 of the day's work be prolonged much beyond eight hours. Indeed, under 

 the management of a good driver, a full day's work may be completed in 

 the time before mentioned — six hours — with benefit to the health and 

 vigour of the animal." 



We tnay be permitted, however, to abandon the idea of improving the 



