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Rh pressure shall exactly coincide with the line of the fulcrum knife-edge; the object of this is that the pressure may not influence the sway of the steelyard, which must depend entirely upon the poise. By means of a pair of mine wheels the small spur wheel causes a screwed shaft, which runs along the middle of the steelyard, to revolve, and as it revolves it carries the large poise along the steelyard.

Thus, if the poise be at the zero end of the steelyard at the left-hand side of the machine, when the load comes upon the platform the screwed shaft carries the poise along the steelyard till equilibrium is established, and the end of the steelyard drops. By the first part of this drop the movement of the poise is suddenly stopped, as will be explained below, and the travel of the poise along the steelyard, which measures the load on the platform, is recorded by the amount of rotation of the large spur wheel, and this is suitably shown on a dial in connexion with the wheel. By the second part of the drop the motion of the poise is reversed and the poise is run back to the zero end in readiness for the next load.

All of this is effected automatically as follows:—

The machine is driven continuously by a belt from a motor which wraps round the large drum at the right-hand side of the machine. On the same axle as the drum and behind it is a small pulley which is keyed upon the axle and is connected with the small pulley (which runs idle on its shaft) at the left-hand side of the machine by a crossed belt. Thus these two small pulleys are always running, but in opposite directions. The drum-shaft is connected by a friction

clutch with a shaft in the same line, on which are keyed a sprocket wheel and a ratchet wheel. The sprocket wheel is connected by a chain with a similar sprocket wheel which is keyed on the same shaft as that of the left-hand pulley. The ratchet wheel is acted upon by a pawl which is shown on the diagram. When the poise is at the zero end, and there is no load on the platform, the end of the steelyard is down, and has locked the ratchet wheel by means of the pawl; the shaft being thus locked, the sprocket wheels are stopped, the drum-shalt runs free by the friction clutch, and the two pulleys which are connected by the crossed band are running idle. When the load to be weighed comes upon the platform, the end of the steelyard rises and unlocks the ratchet wheel through the pawl; the sprocket gearing is driven by the friction clutch, and drives the axle of the left-hand small pulley. The metre wheels come into operation and the poise is carried along till the end of the steelyard drops, and locks the ratchet wheel. By means of a horizontal rod the same drop of the steelyard also locks together by clutch gearing the left-hand pulley and the adjacent sprocket wheel, and the pulley drives the sprocket wheel in the opposite direction to that which it had before. Consequently the motion of the metre wheels is reversed and the poise is run back to zero. When the poise arrives at zero it frees the clutch which connects the pulley and the sprocket wheel, and the machine is then ready for the next load. The poise having arrived at the end of its run and unable to go further, the metre wheels and the sprocket gearing are stopped, and the two pulleys and the cross belt run idle till the next load comes upon the platform.

Automatic Luggage Weighing Machine (fig. 20).—This machine is intended for the weighing of personal luggage at railway stations. It consists of a platform which is carried by levers arranged in the manner of an ordinary platform machine, which are connected with the registration mechanism by a vertical rod. This rod is continued upwards by a pair of thin nickel bands which are led right and left over two horizontal cylinders, round which they partly wrap, and to which they are firmly attached. The diameter of the middle part of the cylinders is greater than that of the ends, and the bands from the vertical rod are led over the middle part. To each cylinder a pair of similar nickel bands are led downwards from the top of a casting which is bolted to the frame. The lower ends of these bands pass round the under side of the end portions of the cylinders, wrapping close round them, and are firmly attached to them. To the bottom of each cylinder is rigidly attached a heavy solid cylinder of lead, and these are the regulators of the position of equilibrium of the cylinders when they rotate under the action of the load. When the load comes upon the platform the pull of the vertical rod is transmitted by the nickel bands to the cylinders around which they are wrapped, and causes them to revolve. As they rotate they roll themselves up the pairs of bands which are attached to the top of the casting, and at the same time cause the leaden weights attached to the bottoms of the cylinders to take up a lateral position, where they exercise a leverage opposing the motion of the cylinders, and bringing them up in a definite position corresponding to the pull of the vertical rod. By the rolling of the cylinders up the vertical bands from the casting the cylinders are raised vertically through a space defined by the position of the leaden regulators. By means of suitable and simple mechanism this vertical movement of the cylinders works plunger pistons in a pair of cylinders which contain glycerin, and these deaden the vibrations of the machinery while weighing is going on. The same vertical movement also actuates the index finger of a large-dial, on which the weight of their luggage can be easily read by passengers standing near while their luggage is being weighed.

.—Julius Weisbach, Mechanics of Machinery and Engineering (London, 1848); Ernest Brauer, Die Konstruktion der Waage (Weimar, 1887); H. J. Chaney, Our Weights and Measures (London, 1897); Airy on "Weighing Machines," ''Proc. Inst. C .E.'' vol. cviii.; W. H. Brothers on "Weighing Machinery," ''Trans. Soc. Engineers'', vol. for 1890.

WEIGHTS AND MEASURES. This subject may be most conveniently considered under three aspects—I. Scientific, II. Historical; and III. Commercial.

1. Units.—In the United Kingdom two systems of weights and measures are now recognized—the imperial and the Metric System. The fundamental units of these systems are—of length, the yard and metre; and of mass, the pound and kilogram.

The legal theory of the British system of weights and measures is—(a) the standard yard, with all lineal measures and their