_Description of the Instrument._--The farmer's barometer, as figured in the margin, consists of an upright tube of mercury inverted in a cistern of the same fluid; this is secured against a strong frame of wood, at the upper end of which is fixed the scale, divided into inches and tenths of an inch. On either side of the barometer, or centre tube, are two thermometers--that on the left hand has its bulb uncovered and freely exposed, and indicates the temperature of the air at the place of observation; that on the right hand has its bulb covered with a piece of muslin, from which depend a few threads of soft lamp cotton; this cotton is immersed in the small cup situated just under the thermometer, this vessel being full of water; the water rises by capillary attraction to the muslin-covered bulb, and keeps it in a constantly moist state. These two thermometers, which we distinguish by the names "Wet Bulb" and "Dry Bulb," form the Hygrometer; and it is by the simultaneous reading of these two thermometers, and noting the difference that exists between their indications, that the humidity in the atmosphere is determined. Admiral FitzRoy's words (see p. 22) are placed upon the scale of the barometer, as the value of a reading depends, not so much on the actual height of the mercury in the tube, as it does on whether the column is rising, steady, or falling. The moveable screw at the bottom of the cistern is for the purpose of forcing the mercury to the top of the tube when the instrument is being carried from place to place, and it must always be unscrewed to its utmost limit when the barometer is hung in its proper place. After this it should never be touched. The manner in which the Hygrometer acts is as follows: It is a pretty well-known fact that water or wine is often cooled by a wet cloth being tied round the bottle, and then being placed in a current of air. The evaporation that takes place in the progressive drying of the cloth causes the temperature to fall considerably below that of the surrounding atmosphere, and the contents of the bottle are thus cooled. In the same manner, then, the covered wet bulb thermometer will be found _invariably_ to read lower than the uncovered one; and the greater the dryness of the air, the greater will be the difference between the indications of the two thermometers; and the more moisture that exists in the air, the more nearly they will read alike. The cup must be kept filled with pure water, and occasionally cleaned out, to remove any dirt. The muslin, or cotton-wick, should also be renewed every few weeks. The hygrometer may be had separate from the barometer, if the combined instruments cannot be sufficiently exposed to the external air, this being essential for the successful use of the hygrometer.