De Luc, who devoted his attention to the same object has followed a different method. This philosopher employed for the construction of his hygrometers, a very thin slip of whale-bone, which per forms the same office as .the hair in the hygrometer of Saussure. He kept this whale-bone bent by means of a spring, the action of which he preferredto that of a weight: he determined the degree of extreme humidity, by immersing the slip of whale-bone entirely under water ; and to fix the opposite limit, which is that of extreme dryness, he made use of calcined lime, which he inclosed with the hygro meter under a glass bell. The choice of lime is founded on this, that the calcina tion having produced a higher degree of dryness, if it be afterward left to cool, so far that it may be placed without incon venience under the glass bell destined for the experiment, it will be still found, as to sense, in the same state of dryness, since it is very slow in acquiring humidi ty; and thus all its absorbent faculty will be employed to dry up, by little and little, the air contained under the receiver, and to make the hygrometer itself pass to a state which approaches the nearest pos sible to extreme dryness. The hygrome ter has been long neglected in meteoro logical observations; it is necessary to as sociate with it the thermometer and the barometer, to be in a state to unravel the complication of different causes which in fluence the variations of the atmosphere ; and it is only by the aid of a long series of observations, made by these various in struments, together with all the indica tions are deduced from the state of the heavens, that we can obtain such data as will enable, us to prognosticate, with great probability, the temporary changes, and to arrive at a plausible the ory upon this subject, so interesting, and so naturally calculated to excite our curi osity.
Mr. Marshall says, that a simple instru ment'of this sort may be formed bymeans of "a flaxen line (large well manufactur ed whip cord) five feet long, and having a graduated scale fixed to an index, mov ing on a fulcrum. The length of the in dex, from the fulcrum to the point, should be ten inches; that of the lever, from the fulcrum to the middle of the eye, to which the cord is fixed, two and a half." He adds, that "the principle on which• this hygrometer acts is obvious. The air becoming moist, the cord imbibes its moisture; the line, in consequence, is shortened; and the index rises. On the contrary, the air becoming dry, the cord discharges its moisture, — lengthens, and the index falls. It may be true," he says, " that no two hygrometers will keep pace with each other sufficiently to satis fy the curious. He will venture to say,
however, from seven months close atten tion, that two hygrometers, on this sim ple construction, have coincided suffi ciently for the uses of agriculture. It is true," he adds, "they diminished in the degree of action ; but as the scale may be readily diminished in extent, and as a fresh line may be so cheaply and so readi iy supplied, this is not a valid object ion." It is remarked, that "this diminution in the degree of action depends consider ably on the construction; the propriety, or rather delicacy, of which rests, almost solely;-on this point : the weight of the index should be so proportioned to the weight of the lever and cord, that the cord may be kept perfectly straight, with out being too much stretched. He made one with. a long heavy index, and in or der to gain a more extensive scale with a short lever ; but, even when it was first put up, it could barely act; and in a few weeks it flagged, and was not able to raise the index, though the air was un commonly moist. He therefore made another with the same length, both of in. dex and lever, but with a lighter index, and a heavierlever, so as to gain the pro portion above and it has act ed exceedingly well." He thinks that no farmer, " who wishes to profit by the hygrometer, should have less than two. Three or four would be more advisable. They would then assist in correcting each other; and, in case of renewal or alteration, there would be no danger of losing the state of the atmosphere ; which, if only one is kept, must neces sarily be the case. The principle on which this hygrometer is formed is not, he says, confined to a small cord, and an index of ten inches long: it may be ex tended to a rope, of any length or thick ness, and to an index and scale of almost any dimensions and extent." But one, or more, on a portable construction, might, he thinks, be found useful. An axe is the form he has thought of; the edge, gradu ated, will constitute the scale'; and the handle will receive the cord : this may be hung up in the shade, exposed to the action of the air; or, by means of a spike in the end of the handle, it may be placed in the open field. By placing it on fallow ground, it may be actuated by the per spiration of the earth ; among vegetables by vegetable perspiration ; by the means of one, or, more probably, by the means of several, placed at varied heights, the different degrees of moisture at different altitudes may be ascertained, &c. In fact, he considers the hygrometer, whether it is a prognostic of the weather or not, as a most valuable oracle to the farmer. See WEATHER.