the best-known current-meters is Pitot's tube (fig. 4), which acts by the ascension of water in a bent tube whose fun nel-shaped lower end is presented squarely to the current, the indications being read by a float or graduation on the vertical part of the tube. \Volt mann's dynamometer current-gauge (179o) consists of several spiral vanes on a shaft carrying an endless screw, which rotates a series of geared wheels and a register. On being placed in a current, the vanes assume a position perpendicular to the flowing water, and their rotation actuates the clock-work mechanism of the register by which is indicated the velo city in miles per hour, the rate or force being deducted from the rotation of a given time. These current-gauges, being dependent on the velocity of the liquid in motion, all partake essentially of the nature of dynamom eters or force-measuring machines. They are, however, used to determine the quantity of water carried by any given current, the velocity of the moving water being obviously the main factor in the determination of the quantity. Figure 5 exhibits a current-meter for use in small rivers and streams to show the rate of flow of the tide, or the number of gallons flowing from a reservoir.
tide-gauge, also called thallassometer and mari graph, is a device used in harbors to measure the rise and fall of the tides. A simple form of tide-gauge is represented by the graduated spar, whose lower end is suitably secured in the bed of a stream, the gradua tions indicating the height of the water. An arrangement similar to the tide-gauge is the milometer, for measuring the rise of the Nile during its periodical floods. It consists of a pillar, 16 cubits high, marked with the necessary divisions for ascertaining the proportionate increase or decrease of the flood. The rise of the Nile has always been a subject of great anxiety to the Egyptians, for upon the periodical saturation of the ground and the coating of soil left upon it depends the success of the year's hus bandry. From the earliest historic period nilometers have been placed in prominent positions in Egypt to enable the officers to watch the rise of the Nile, which was proclaimed by criers; and formerly the amount of taxation to be imposed upon the country was determined by this means. The nilometer at Cairo has been erected for many centuries, but it is not so ancient as the one at Elephanta, which consists of a staircase descend ing to the Nile between two walls. One wall has engraved on it a series of marks representing the height to which the water has risen on certain occasions.
Sclire:cistering have a mechanism by which the ebb and flow of the tides are automatically registered or indicated. The self-regis tering gauge of Sir Thomson (pl. 122,fig. 6) is described as follows: Supported by a stand is a clock which, besides keeping time, actuates be neath it a drum whose rotation winds upon itself, with a uniform movement, a continuous web of paper from another cylinder. In position on the stand alongside the clock are two pulleys, one of small and the other of com paratively large diameter. Passing over the large pulley is a cord, to whose lower end is fastened a counterbalanced float-weight which rises and falls with the tide. On one end of the pulley-shaft is a pinion which
engages a gear-wheel on the shaft of the small pulley, from which is sus pended a weighted ink-bottle carrying a pen. These rise and fall with the movement of the float, and the pen traces a continuous record on the paper around the drum in the form of zigzag lines whose angles corre spond with high-water and low-water periods, the horizontal distance between each two angles indicating the intervals between two high or two low tides.
as distinguished from hydrometers, are devices for mechanically measuring the amount of water or other liquid received or discharged through an orifice. The forms of such meters are various, and they may be divided into seven classes: (i) those in which the fluid passes through a horizontally-revolving case with a peripheral discharge, or a stationary case containing a horizontal turbine-like wheel which delivers a known amount of water at each rotation; (2) those which are operated by the pressure of the water acting on rotary pistons whose action is on the principle of the rotary engine (p. 26o), and which are the converse of the rotary pump (p. 332); (3) the Archimedian screw (p. 327); (4) those with a reciprocating piston in a cylinder of known capacity, on the prin ciple of the ordinary steam-engine; (5) the meter-wheel; (6) the pulsat ing diaphragm, which displaces the water from its respective sides alter nately; and (7) the bucket-and-balance, or those in which reservoirs of known capacity on the respective ends of the beam are alternately presented to catch the water and are depressed and emptied as they become filled.
Figures 7 and S exhibit a water-meter whose principle of action is the reciprocation of pistons or plungers. The machine consists of two parallel cylinders, iu each of which are two piston-heads connected by a stem and acting as a double plunger. The space between the pistons is filled with water and contains the mechanism that actuates the water valves, which are slide-valves. Under the stem or central space of each piston is placed the slide-valve of the opposite piston, so that the motion of each piston actuates the valve that admits water against the other pis ton, the action being reciprocal. Each stroke of the pistons is marked ou a counter or register; the meter is therefore of the positive kind, and furnishes no water unless working properly. The counter, attached to the top of the meter and operated by a ratchet movement, registers in cubic feet—one foot being 7A5 gallons, U. S. standard—and is read in the same way as the registers of gas-meters, the dials recording tens, hun dreds, thousands, ten thousands, hundred thousands, and millions of cubic feet. The motion of the pointers is not continuous, as they move a little and stop, and again move at the next stroke of the plungers; the flow of water, however, is continuous. The inlet-pipe, which is not shown in the Figures, is in a line with the eduction-pipe and on the opposite side of the meter. The machine measures with great accuracy the quantity of water or other liquid passing through it.