The common turnpike weighing machine is a compound steelyard (see BALANCE), and is used for ascertaining the weight of loaded carts. The apparatus, of which a ground-plan is here given (fig. 1), is arranged in a box A B C D, which is about 12 inches deep, and is sufficiently sunk in the ground to place the highest part of the completed machine on a level with the line of road. A fixed wooden bridge, by which the horse may pass across the machine, is laid over the middle part of the box. At each side of this bridge is a metal wheel-track. On these the cart is rested by its two wheels and a prop-staff. The tracks are fixed to and borne up by a platform (or skeleton frame) placed below the wooden bridge, but above the works. The platform has four short feet, the lower ends of which are formed into small hemispherical cavities. These cavities rest at E F G H, on pointed studs about an inch long. The two studs at E and F are fixed on the crooked lever A K B, with their points upward, and the two at G and H are similarly fixed on the companion lever D I C. Sharpened studs, also about an inch long, project downward from the four extremities of the crooked levers. The points of these studs rest in small hemispherical cavities, formed at the upper ends of blocks which are fixed as bearers for them in each corner of the box. The short parts of the crooked levers parallel to O L, and near the middle of the box, have their under edges sharpened, and rest on the angle of a sustaining fulcrum I K, which passes through, and is fixed to the lever O M. This lever is also crossed at L by the fulcrum which forms the common centre of movement for the whole machine. The angle of the fulcrum points downward, and rests, as in the ordinary balance, on concave arcs, situated one on each side of the lever O M. These arcs are formed on the upper end of a block, which rises from and is fixed to the bottom of the box. The lever is again crossed at M, by a fulcrum having its angle pointing upward; and from this angle a receiving-scale for the weights is hung by a shears or double ring. In cases where it is wished that the machine when unloaded should preserve equilibrium, the lever O M is prolonged to the left, and loaded at that end, to the extent requisite to balance the weight of the levers.

A side view of B K, one of the limbs of the crooked lever A K B is here given, in the direction of the diagonal B D. The lever, as shown in fig. 2, is kneed to an extent sufficient to admit of the angular points of the fulcra being placed in the same horizontal line. The other limb, A K, is of the same shape. The form of the entire lever, A K B, is thus easily understood. Its companion lever, D I C, is of the same construction. The limbs of the levers are therefore so shaped that the fourteen sharp bearing-parts of all the fulcra are situated, during equilibrium, in one horizontal plane. These bearing-parts, and the parts they bear against, are all formed of polished steel.

If now E A be supposed a tenth part of the length of E K, then 100 lb. placed on the platform, or on the wheel-tracks, and pressing, therefore, on the studs E F G H, will