249

I be understood, except by those who are experienced inmatters of this nature. Such, however, as comprehendbut a little of what I have written, would, if they saw theinstrument, be compelled to acknowledge the skill exhi-bited in its contrivance.

CHAPTER XIY.

OF MEASURING A JOURNEY.

Let us now consider an invention by no means useless,and delivered to us by the ancients as ingenious, andby means of which, when on a journey hy land or sea,one may ascertain the distance travelled. It is as fol-lows. The wheels of the chariot must be four feet dia-meter ; so that, marking a certain point thereon, whenceit begins its revolution on the ground, when it has com-pleted that revolution, it will have gone on the road overa space equal to twelve feet and a half. This being ad-justed on the inner side of the nave of the wheel, let adrum-wheel be securely fixed, having one small toothprojecting beyond the face of its circumference; and inthe body of the chariot let a small box be fastened, witha drum-wheel placed to revolve perpendicularly, and fas-tened to an axle. The latter wheel is to be equallydivided, on its edge, into four hundred teeth, corre-sponding with the teeth of the lower drum-wheel; besidesthe above, the upper drum-wheel has on its side onetooth projecting out before the others. Above, in anotherenclosure, is a third horizontal wheel toothed similarly,and so that the teeth correspond with that tooth which isfixed to the side of the second wheel. In the third wheeljust described are as many holes as are equal to thenumber of miles in an usual day’s journey. It does not,

m 3