The most simple form of the hygrometer was formerly a very favourite indicator of the state of the weather, and usually consisted of the figure of a monk with his hood, which is attached to a bit of catgut; this covering of paper, painted to represent the hood, falls over the head on the approach of damp weather, and inclines well back during the period that the air is dry or contains less moisture; and simple as it is, this hygrometer, in conjunction with the reading of the barometer, may assist Paterfamilias in deciding the fate of a pet bonnet or velvet mantle, which is or is not to be worn on a doubtful day. (Fig. 378.)
Fig. 378.
The monk hygroscope, in which the hood, a b, covers the head to dotted line c in wet weather, and takes various intermediate positions, being quite back and on the shoulders in dry states of the air. A thermometer, d, is usually attached.
A decision on the possible changes of the weather requires considerable experience, and it has been said that one of the most celebrated marshals of France owed his invariable success in military combinations and attacks to his attention to the signs of the weather, as indicated by the state of the air during the phases of the moon. Inexperienced persons (and by that we mean young persons) may, however, take a certain position in the rank of "weather prophets" by consulting the weathercock, the barometer, and the hygrometer, before committing themselves to an opinion, if asked to say what the weather will be.
The dry and wet bulb hygrometer (as represented in the next engraving) consists of two parallel thermometers, as nearly identical as possible, mounted on a wooden bracket, one marked dry, the other wet. The bulb of the wet thermometer is covered with thin muslin, round the neck of which is twisted a conducting thread of lamp-wick, or common darning-cotton; this passes into a vessel of water, placed at such a distance as to allow a length of conducting thread of about three inches; the cup or glass is placed on one side, and a little beneath, so that the water within may not affect the reading of the dry bulb thermometer. In observing, the eye should be placed on a level with the top of the mercury in the tube, and the observer should refrain from breathing whilst taking an observation. The temperature of the air and of evaporation is given by the readings of the two thermometers, from which can be calculated the dew-point, tables being furnished for that purpose with the instrument. (Fig. 379.)
Fig. 379.
The dry and wet bulb hygrometer.
The colour of the sky at particular times affords the most excellent guidance to doubting members of pic-nic or other out-of-door parties. Not only does a rosy sunset presage fine weather, and a ruddy sunrise bad weather, but there are other tints which speak with equal clearness and accuracy. A bright yellow sky in the evening indicates wind; a pale yellow, wet; a neutral grey colour constitutes a favourable sign in the evening, an unfavourable one in the morning. The clouds, again, are full of meaning in themselves. If their forms are soft, undefined, and feathery, the weather will be fine; if their edges are hard, sharp, and defined, it will be foul. Generally speaking, any deep, unusual hues betoken wind or rain, while the more quiet and delicate tints bespeak fine weather.