Page:Blackwood's Magazine volume 002.djvu/668

Leslie, I refer your readers to that author's treatise on heat and moisture, to the article Hygrometry in the Edinburgh Encyclopædia, or to the first volume of your Magazine, p. 381. the modification of it which I am now to propose, the principal is the same, though the form is somewhat different.

CD (fig. 1.) is a tube, such as is commonly used for constructing a self-registering thermometer, bent upwards at C and D, and terminating in a bulb A. Into this bulb is introduced a portion of sulphuric acid, sufficient to fill the tube and a small part of the bulb; and along with the acid a small bit of glass, a, of such a diameter as to move easily in the tube when the instrument is inverted. To the extremity, D, another bulb, B is attached; and the air contained in both bulbs is so adjusted, that when they are at the same temperature, the liquid stands at a point near the extremity D, and which is marked 0 on the attached scale E F. If the temperature of the bulb B be now increased, or, which is the same thing, if that of A be diminished, the portion of air in the upper part of the bulb will contract, while that contained in B will expand in the same proportion, and the liquid will of course be forced from D towards C. In the scale adopted by Professor Leslie, the distance between the freezing and boiling points is divided into a thousand equal parts, and is hence denominated the millesimal. In this climate, however, a tenth part of that scale, or one hundred degrees, will embrace the greatest range of the instrument; and that point may be thus obtained:—Let the bulb A be surrounded with melting snow, while the instrument is placed in an atmosphere of the temperature 50, and let the point be marked at which the liquid becomes stationary. The distance between zero and this point will then be 18 degrees of Fahr. or 100 of the millesimal scale; and that distance being divided into an hundred equal parts, will give the graduation required. To prepare the instrument for observation, it only remains to cover the bulb A with silk, and moisten it, taking care that the two bulbs be as nearly as possible of the same colour. The index, or small bit of glass, a, is then to be brought to the extremity of the liquid, by depressing the extremity D, and the instrument to be exposed in a horizontal position. As the evaporation from the surface of the bulb A goes on, the air within contracts, from the depression of temperature produced by the evaporation; and the liquid is forced from D towards C by the elasticity of the air in B, carrying with it the index a. When the evaporation has reached its maximum, the liquid, as well as the index, becomes stationary; but should the process of evaporation diminish, the liquid will again move towards D, while the index is left behind, thus marking the maximum of dryness in the absence of the observer.

To find the greatest degree of moisture, another instrument is to be employed, which is represented in fig. 2. The only difference between this and the former is, that the air in the two bulbs is to be so adjusted, that, when they are at the same temperature, the liquid may stand near the extremity C, the distance between C and zero being a little more than the length of the index a; and the bulb B is to be covered as A was in the former. The scale is graduated as before. When the instrument is adjusted and exposed, evaporation goes on from the surface of B; and the air within being there-