An instrument for ascertaining the degrees of dryness or moisture of the atmosphere; therefore whatever substance expands by mois-ture or contracts by dryness, may be employed for the purpose, in connexion with a suitable index and scale, showing the changes it undergoes. The hygrometer invented by Saussure consisted of a hair, divested of its oil by boiling it in water containing one per cent, of sulphate of soda. One of the ends of the hair was attached to a fixed point, and the other to the circumference of a movable cylinder, that carries a light index; the hair being kept straight by a counterweight of three grains, suspended by a fine silk thread to the cylinder, and wound thereon the contrary way of the hair. As the hair lengthens or shortens by changes in the humidity of the air, the cylinder is put in motion, and the index fixed thereto points out on a graduated circle the degree. This pretty little contrivance of Saussure's was regarded as a faithful indicator of the true condition of the atmosphere until M. de Luc proved that hair was incapable of becoming a correct measurer of humidity, and that this was owing to its organic reticular structure.

The last-mentioned philosopher made an hygrometer, in which ivory was employed as the medium of exhibiting the change of humidity. As ivory expands by moisture and contracts by dryness, he formed a very thin hollow cylinder of this substance, open only at the upper end, into which he fitted the open end of a very fine long glass tube, like that of a thermometer. Into these he introduced a quantity of quicksilver, filling the ivory cylinder, and a part of the glass tube. The consequence is this; when moisture swells the ivory cylinder, its capacity increases, and the mercury sinks in the glass tube; and vice versa, when the air is drier, the ivory contracts, and forces the mercury higher up the glass tube. As this instrument is susceptible of being influenced by heat and cold, like the thermometer, as well as by that of humidity, its indications are not to be depended upon. Hygrometers are constructed, in a great variety of ways, by means of sponge, and a rod suspended like a scale-beam. The rod is to have one of its ends pointed, to serve as an index, and at the other end a hook to which the sponge is fastened.

The sponge is prepared by first washing it thoroughly in clean water, and when it is dry washing it again in either water or vinegar, in which a quantity of salt of tartar or sal-ammoniac has been dissolved; after which, the sponge being well dried, is fit for use. Then having fixed against that part of a wall over which the point of the index will traverse, a graduated circular arc, the index will show on this scale the state of the atmosphere; for when the air is humid, the sponge will imbibe moisture from it, become heavier, and consequently pull that arm to which it is suspended downwards; while the other arm or index will move upwards, along the graduated arc on the wall. On the contrary, when the air becomes drier, it imbibes the moisture from the sponge, which consequently becoming lighter, the index preponderates, and moves down the graduated arc, thus showing the moisture or dryness of the atmosphere. Instead of the sponge, Mr. Gould recommends oil of vitriol, which grows sensibly heavier or lighter, as the moisture of the air increases or decreases; so that being saturated in the dampest weather, it retains, loses, or resumes its acquired weight, with the continuation, decrease, or increase of the moisture in the air.

So great is the alteration in the weight of this liquor from the above cause, that in the space of fifty-seven days, it has been known to change its weight from three to nine drams, and has shifted the tongue of a balance thirty degrees. The curious on the subject of hygrometers may meet with a great multiplicity of them in the Philosophical Transactions, and the various scientific journals.