certain number of millimetres traversed by the style in the vertical direction. The course of the style, which corresponds to a kilometre, ought to be experimentally determined for each vehicle, for the perimeter of wheels is not always the same. But it is clear that, if from each kilometre-stone to another we obtain five millimetres, for example, for the course of the style, this length will always be found to be traversed each kilometre by the same vehicle. Our apparatus is then a measurer of distances, and dispenses with the necessity of attending to
Fig. 7.—Tracings of the Odograph: a, rapid coach with (stoppages; b, slow coach; c, gas meter, frequency of turns of the wheel; d, curve of the turns of a clock wheel-work with fly.
the existence of kilometre-stones; it enables the distance traversed on any road whatever to be estimated, and even when there is no beaten track. Thus in a journey of discovery we may measure the distance traversed by a cart. To remain in the conditions of ordinary life, have we not sometimes in the country a choice of two or three roads to go from one place to another? To know which is the shortest we appeal to the watch, as if the least duration of a walk corresponded to the least distance. The odograph will give in this respect very precise information.
There are again a great number of questions which we ask daily without being able to solve them. Does such a draught-horse go quicker than such another? Does this trot better to-day than yesterday? By increasing the ration of oats do we increase speed? Compare the slope of two curves of rates, and you will have the reply to all these questions without being obliged to make special experiments on a measured road, watch in hand.
It is not only to the speed of vehicles that the registering apparatus applies; it traces, though with less precision, the rate of progress of
