If you wish to be convinced of the advantage of this mode of traction, 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.
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 which 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 progress 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, otherwise 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 comparative 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-
