is an instrument of so much importance to the meteorologist, that it becomes us to give some account of every improvement of it which has fallen under our notice. In the Encyclopædia, the principles upon which hygrometers are constructed have been clearly stated, and the defects of each kind of hygrometer pointed out.
Instead of hairs or cat-gut, of which hygrometers of the first kind are commonly made, Cassébois, a Benedictine monk at Mentz, proposed to make such hygrometers of the gut of a silk worm. When that insect is ready to spin, there are found in it two vessels proceeding from the head to the stomach, to which they adhere, and then bend towards the back, where they form a great many folds. The part of these vessels next the stomach is of a cylindrical form, and about a line in diameter. These vessels contain a gummy sort of matter from which the worm spins its silk; and though they are exceedingly tender, means have been devised to extract them from the insect, and to prepare them for the above purpose. When the worm is about to spin, it is thrown into vinegar, and suffered to remain there twenty-four hours; during which time the vinegar is absorbed into the body of the insect, and coagulates its juices. The worm being then opened, both the vessels, which have now acquired strength, are extracted; and, on account of their pliability, are capable of considerable extension. That they may not, however, become too weak, they are stretched only to the length of about fifteen or twenty inches. It is obvious that they must be kept sufficiently extended till they are completely dry. Before they attain to that state, they must be freed, by means of the nail of the finger, from a slimy substance which adheres to them. Such a thread will sustain a weight of six pounds without breaking, and may be used for an hygrometer in the same manner as cat-gut; but we confess that we do not clearly perceive its superiority.
To an improvement of the hygrometer constructed on the third principle, stated in the Encyclopædia, M. Hochheimer was led in the following manner:
Mr Lowitz found at Dmitriewik in Africam, on the banks of the Wolga, a thin bluish kind of slate which attracted moisture remarkably soon, but again suffered it as soon to escape. A plate of this slate weighed, when brought to a red heat, 175 grains, and, when saturated with water, 247; it had therefore imbibed, between complete dryness and the point of complete moisture, 72 grains of water. Lowitz suspended a round thin plate of this slate at the end of a very delicate balance, fastened within a wooden frame, and suspended at the other arm a chain of silver wire, the end of which was made fast to a sliding nut that moved up and down in a small groove on the edge of one side of the frame. He determined, by trial, the position of the nut when the balance was in equilibrium and when it had ten degrees of over-weight, and divided the space between these two points into ten equal parts, adding such a number more of these parts as might be necessary. When the stone was suspended from the one arm of the balance, and at the other a weight equal to 175 grains, or the weight of the stone when perfectly dry, the nut in the groove shewed the excess of weight in grains when it and the chain were so adjusted that the balance stood in equilibrium. A particular apparatus on the same principles as a vernier, applied to the nut, shewed the excess of weight to ten parts of a grain.
Lowitz remarked that this hygrometer in continued wet weather gave a moisture of more than 55 grains, and in a continued heat of 113 degrees of Fahrenheit only 1 degree of moisture.
The hygrometer thus invented by Lowitz was, however, attended with this fault, that it never threw off the moisture in the same degree as the atmosphere became drier. It was also sometimes very deceitful, and announced moisture when it ought to have indicated that dryness had again begun to take place in the atmosphere. To avoid these inconveniences, M. Hoehnheimer proposes the following method:
1. Take a square bar of steel about two lines in thickness, and from ten to twelve inches in length, and form it into a kind of balance, one arm of which ends in a screw. On this screw let there be screwed a leaden bullet of a proper weight, instead of the common weights that are suspended.
2. Take a glass plate about ten inches long, and seven inches in breadth, destroy its polish on both sides, free it from all moisture by rubbing it over with warm ashes, suspend it at the other end of the balance, and bring the balance into equilibrium by screwing up or down the leaden bullet.
3. Mark now the place to which the leaden bullet is brought by the screw, as accurately as possible, for the point of the greatest dryness.
4. Then take away the glass plate from the balance, dip it completely in water, give it a shake that the drops may run off from it, and wipe them carefully from the edge.
5. Apply the glass plate thus moistened again to the balance, and bring the latter into equilibrium by screwing the leaden bullet. Mark then the place at which the bullet stands as the highest degree of moisture.
6. This apparatus is to be suspended in a small box of well dried wood, sufficiently large to suffer the glass plate to move up and down. An opening must be made in the lid, exactly of such a size as to allow the tongue of the balance to move freely. Parallel to the tongue apply a graduated circle, divided into a number of degrees at pleasure from the highest point of dryness to the highest degree of moisture. The box must be pierced with small holes on all the four sides, to give a free passage to the air; and to prevent moisture from penetrating into the wood by rain, when it may be requisite to expose it at a window, it must either be lacquered or painted. To save it at all times from rain, it may be covered, however, with a sort of roof fitted to it in the most convenient manner. But all these external appendages may be improved or altered as may be found necessary.