322 



THE POPULAR SCIENCE MONTHLY. 



If you wish to be convinced of the advantage of this mode of trac- 

 tion, yoke yourself to a hand-barrow by means of a rigid leather strap, 

 such as you see used in the streets of Paris or London, where too often 

 man is employed to drag burdens. When you have well noted the 

 painful shocks which this mode of traction transmits to the shoulders, 

 place between the strap and the barrow the elastic tractor and repeat 

 the experiment. After, that no doubt is possible ; the shoulders are 

 no longer bruised by the shaking of the pavement, and a comfort is 

 experienced which will evidently be experienced in the same degree 

 by a horse placed in conditions of elastic traction. 



Ifttfcttfifc 



o >c 



Fig. 2. Tracing of the dynamograph for a vehicle drawn by a horse. 



To obviate suffering of men and animals is unfortunately not a 

 motive sufficient to induce everybody to modify the old system of 

 harnessing. To certain minds known as positive, it is necessary to 

 prove that elastic traction has economical advantages, and that a horse 

 thus harnessed is able to draw heavier loads. This fact, which results 

 from the experiments you have seen, requires to be rigorously proved 

 by the aid of the graphic method. It is to the genius of Poncelet 

 that we owe the record of work expended by different motors. 



Everybody knows what a dynamometer is, viz., a spring which, 

 yielding to tractions exerted upon it, is deformed in proportion to the 

 efforts developed. Let us adapt to a spring of this kind a pencil 

 Avhich touches a strip of paper, and let us so arrange things that the 

 movements of the wheel of a carriage shall impress upon the paper 

 a motion of translation. While the effort of traction of the horse will 

 communicate to the spring movements more or less extended, the prog- 

 ress of the carriage will draw out the paper, and from these combined 

 movements will result a curve (Fig. 2), which can be resolved into 

 a series of ordinates or vertical lines in juxtaposition, expressing by 

 their unequal heights the series of efforts resulting from each element 

 of the road traversed. The sum of these elementary efforts, other- 

 wise the surface of paper limited in height by the flexures of the curve, 

 will be the measure of the work expended. If we record in a compara- 

 tive manner the work done by the same vehicle harnessed with rigid 

 traces or supplied with elastic tractors, we see (Figs. 3 and 4) that the 

 area of the curve is greater, that is, that there has been more work 

 expended, while rigid traces have been used. In the most favorable 

 cases that I have met with, the economy of work by elastic traction 

 has been twenty-six per cent. 



But, it may be objected, the recording dynamometer itself consti- 



