MEASUREMENT OF LIQUIDS AND GASES.
Appliances for measuring liquids may be divided into two classes: (1) those for determining the density of liquid bodies, and (2) those for deter mining their quantity. Such appliances are called mders. The terms meter, register, counter, and indicator are frequently used interchange ably, but they arc not exactly synonyms. An indicator gives audible or visible notice of results or conditions; a register records; and a meter measures distance, capacity, power, etc., giving results in volumes or other units, and it may have an indicator for reading, or a register for recording, its action.
for testing the relative density of liquids are known, in general, as hydrometers, and specifically, according to their respective uses, as alcoholometers, oleometers, lactometers, etc., which terms usually indicate the peculiar kinds of fluids they are designed to test. The hydrometer was known to the ancients, and its invention is ascribed to Archimedes; the principle on which it is based is well illus trated by the familiar experiment which employs the hen's egg for verify ing the strength of lye in soap-making and of brine in curing meats. The common hydrometer consists essentially of an elongated graduated stem com bined with a hollow bulb, and so weighted at its lower end as to cause the stein to remain upright when the instrument is placed in the liquid to be tested (ft/. I22,figs. 1, 2). The zero mark of the scale is adjusted to the point on the stein to which the instrument sinks in distilled water or any other liquid which is taken as a standard. The depth to which it sinks in the liquid to be tested indicates on the graduated stem its specific gravity. As the density of liquids varies with the temperature, in all cases diminishing as the tem perature increases, the graduation of the hydrometric scales must be based on a standard degree of temperature, and this is generally assumed to be 6o degrees. If the liquid at the time of testing is cooler or wanner than that thermometric degree, the indication on the hydrometer must be added to or subtracted from according to a scale which varies with the nature of the liquid. These temperature correlatives are determined by experiment,
and a printed scale for each particular kind of hydrometer may be obtained with the instrument. Hydrometers intended for testing liquids of a wide range of variableness, such as alcoholic liquors, are generally provided with a number of weights to be added to the normal weight of the instru ment where the liquid to be tested is so dense as to buoy the hydrometer beyond the range of the graduated stem. Thus a hydrometer to test whis kies and high wines is usually calculated for liquids containing alcohol from "proof" upward, and when the alcoholic contents of the liquid are below " proof," a weight is added to counterbalance the increased density of the liquid; and this weight is necessarily accounted in connection with the indication on the graduated stem. Some hydrometers are graduated with the zero mark—that is, the point of submersion in distilled water at 6o° temperature—placed midway on the length of the stem, so as to be used for liquids both lighter and heavier than water; and these are techni cally known as " specific-gravity " scales.
Rain annual amount of rainfall is very unequally dis tributed over the earth's surface. This is due to various conditions which exert a definite influence on the amount of precipitation. For measuring the amount of rainfall over a given surface there is employed the rain gauge, variously known as an ombrometer, udometer, pluviometer, etc. A simple device for this purpose consists merely of a funnel, from 5 to 7 inches in diameter, inserted in the neck of a bottle, the rain collected in the bottle being measured in a graduated glass (pl. 122, fig. 3). Self-record ing rain-gauges are very delicate and accurate instruments of more or less complicated construction.