ODOMETER, or WAvwisER, from tz,'.s.n, a road, and frCETCEIY, to measure, is an instrument for measuring the length of road traveller] by a wheel carriage, from the number of rotations performed by the wheel in a given time.
The construction of such an instrument presents no difficulties, the only point of importance being to regis ter the number of revolutions without error, and count them off with facility and accuracy. The odometer which Ive think best fitted for this purpose was invent ed, we believe, by the Rev. Mr. Wollaston, and is re presented in Plate CCCCXX. Fig. 1, where ABC is an arm of steel, having the teeth of an endless screw cut on the end AB. The teeth of this screw engage in the teeth of two wheels K, L, the one L lying behind K ; both its teeth and its circumference being concealed, from view. The wheel K has 100 teeth, and 101 ; and therefore, since the same endless screw turns both wheels, it is obvious that when K has performed one complete revolution of 100 teeth, 100 teeth of L will also have passed forward ; but as L has 101 teeth, the zero of K will have separated from the zero of 1 tooth, and will consequently point to 1 on the scale of L. Af ter a second revolution of K, its zero will point to 2 on the scale of L. IIence when the index of K points to 101 on the scale of L, the endless screw ABC will have performed 10,100 revolutions. When the odometer is to be used, the arm ABC may be fixed to the moveable axle of the carriage.
Mr. Hunter, of Thurston, has proposed to apply the odometer to a light iron wheel, for the purpose of mea suring roads, hedges, walls, and ditches. This applica tion of it is shown in Fig. 2. where A is a light iron wheel two yards in circumference. It is divided into feet by six spokes, one of which is painted white. The handle B is divided at C like a fork, and embraces each end of the axis of the wheel by its elasticity. There is a hole through the middle of the axis, into which is inserted the end BC of the odometer, which is held fast in its place by the nut C.
The following is the method of using it given by ',Mr.
Hunter. " The wheel K, Fig. 1. shows the number of turns made by the iron wheel A, Fig. 2 ; and the wheel A is divided into six feet, commencing at the white spoke. It is advisable always to commence with thc waywiser set at O. To do this, unscrew the nut I, and draw out the screw BC, when the brass wheels K and L can both be set at 0, and the screw and nut I repla ced. Set the wheel A, Fig. 2, on the ground, with the white spoke undermost, and fix thc waywiser into the wheel by means of the nut C, always observing to put it on the left side, as shown in Fig. 2. at E. At any pe riod of measuring, you can tell exactly how far you have gone, and proceed without again setting the way wiser at O.
Suppose the spoke, No. 2. at the ground, as in the figure, the index NI pointing at 26 of K, and the index of K pointing to 78 of L, then the distance measured is 7826 (Urns of A and 2 feet ; and as A measures 2 yards, 7826 X 2=15652 yards, to which add the 2 feet.
In reading off, particular care must be takcn always to read the large figures (viz. those on the wheel L) first, and afterwards to add the small figures, (viz. those on thc wheel K,) and if the figures on K amount to less than 10, a 0 must be prefixed, so that K shall always show two figures ; for instance, L being at 46, and K at 4, the sum is 4604. The easiest way to guard against error is to read 46, and add the word hundred; thus forty-six hundred and four, and not four thousand six hundred and four. For my own use, I have preferred a wheel measuring 6.6 feet, or 6,6v, which is exactly the tenth part of an English chain, and my iron m heel is divided into 10 parts; so that all my calculations are in decimals. If the waywiscr stands, as before, at 7826, and the spoke No. 2. on the ground, then the measure ment is 7826 turns and -hp or 7826.2 turns, equal to 782.62 chains, equal to 78 262 furlongs, which may also be read 78 fur. 2 ch. 6,26 turns ; so that by using deci mals there is no more calculation required than if the wheel measured one yard.