In a small box, 8 in. by 8 in. by 4 in., a complete set of meteorological instruments have been packed. The lid of the box, by an ingenious arrangement, is made to take off and hang up; on it are permanently fixed for observation, a maximum and minimum, and a pair of dry and wet bulb thermometers. The interior of the box contains a maximum thermometer in vacuo for solar radiation, and a minimum for terrestrial purposes, one of Negretti and Zambra's small pocket aneroid barometers, pedometer for measuring distances, pocket compass, clinometer, and lastly a rain gauge. This latter instrument consists of an accurately turned brass ring having an india rubber body fastened to it to receive the rain, which is measured off by a small graduated glass, also contained in the box. Gentlemen travelling will find this compact observatory all that can be desired for meteorological observations. 156. IMPLEMENTS. The practical meteorologist will find the following articles very useful, if not necessary. They scarcely require description; an enumeration will therefore suffice:--_Weather Diagrams_, or prepared printed and ruled forms, whereon to exhibit graphically the readings of the various instruments to render their indications useful in foretelling weather, &c.;--_Meteorological Registers_, or Record Books, for recording all observations, and the deductions;--_Cloud Pictures_, by which the clouds can be readily referred to their particular classification, very necessary to the inexperienced and learners;--Cyclone Glasses, or Horns, outline Maps with Wind-markers, are also useful, especially in forecasting weather. 157. HYDROMETER. A simple kind of hydrometer is very much used at sea, as "a sea-water test;" and as the observations are usually recorded in a meteorological register or the ship's log-book, it may not be altogether out of place to give a description of it here. [Illustration: Fig. 96.] [Illustration: Fig. 97.] It is constructed of glass. If made of brass, the corrosive action of salt-water soon renders the instrument erroneous in its indications. The shapes usually given to the instruments are shown in figs. 96 and 97. A globular bulb is blown, and partly filled with mercury or small shot, to make the instrument float steadily in a vertical position. From the neck of the bulb the glass is expanded into an oval or a cylindrical shape, to give the instrument sufficient volume for flotation; finally, it is tapered off to a narrow upright stem which encloses an ivory scale, and is closed at the top. The divisions on the scale read downward, so as to measure the length of the stem which stands above the surface of any liquid in which the hydrometer is floated. The denser the fluid, the higher will the instrument rise; the rarer, the lower it will sink.