HYGROMETER

X^he hygrometer of De Luc consists of a very thin slip W" whalebone cut transversely, or acro the fibres, and fetched by means of a spring between two points. One enu is fixed to a bar, while the other acts on the shorter arm of the index of a graduated scale. When the whale Lone absorbs moisture it swells, and its length is increased; as it becomes dry it contracts; and the space over which the index moves by the one or the other of these effects gives the measure of the expansion or contraction, and the corresponding change in the hygrometric state of the atmosphere. The action of this hygrometer aijpears to be more uncertain than that of Sau ure. The hygrometers which have been proposed on the principle of a change of weight arising from the absorption of moisture, are liable to still greater objections. Changes of weight may indeed be measured with great accuracy by the common or torsion balance: but in the present case they are so small, that the particles of dust which are at all times floating in the atmosphere may produce a great alteration in the results. A great variety of substances which attract moisture have been employed, such as sponge, cottcm, bibulous paper, caustic potash, the deliquescent salts, sulphuric acid, .; but the indications which they give are deserving of very little credit. Changes of property indicated by the torsion of cords formed of gut, hemp, cotton, ., and the torsion of certain vegetable fibres, are still more fallacious. 2. Hygrometers on the Principle of Condensation. — The instruments of this cla are of a far more refined nature than those which we have been describing. In order to give an idea of the general principle on which they d'ipend, let us conceive a gla»s jar, having its sides perfectly clean and transparent, to be filled with water, and placed on a table in a room where the temperature is, for example, GOO, the temperature of the water being the same as that of the room. Let us next suppose pieces of ice, or a freezing mixture, to be thrown into the water, whereby the water is gradually cooled down to 55, 50, 45, . degrees. As the proce of cooling goes on, there is a certain instant at which the jar loses its transparency, or becomes dim; and on attentively examining the phenomenon, it is found to be caused by a very fine dew or deposition of aqueous vapour on the external surface 'of the ve el. Ihe precise temperature of the water, and consequently of the ve el, at the instant when tiiis deposition begins to be formed, is called the dew point, and IS capable of being noted with great precision. Now this temperature is evidently that to which, if the air were cooled down, under the same pre ure, it would be completely saturated with moisture, and ready to deposit dew on any body in the least degree colder than itself. The difference, therefore, between the temperature of the air, and the temperature of the water in the ve el when the dew begins to be formed, will afford an indication of the dryne of the air, or of its remotene from the state of complete saturation. But the observation which has now been described is capable of affording far more interesting and precise results than a mere indication of the comparative dryne or moisture of the atmosphere. With the help of tables of the elastic force of aqueous vapour at different temperatures, it gives the means of determining the absolute weight of the aqueous vapour difiused through any given volume of air, the proportion of vapour existing in that volume to the quantity that would be required to saturate it, and of measuring the force and amount of evaporation. The elastic force of aqueous vapour at the boiling point of water is evidently equal to the pre ure of the atmosphere. This may be a umed as corresponding to a column oi mercury 30 inches in height. Mr. Dalton, in the fifth volume oi the Manchester Meinoirs, has given the details of a most valuable and beautiful set of experiments, by which he ascertained the elastic force of vapour from water at every degree between its freezing and boiling points in terms of the column of mercury which it is capable of supporting. As the same experiments have since been frequently repeated, and the different results present all the accordance which can be expected in so delicate an investigation, the tension of vapour at the different temperatures may be regarded as sufficiently well determined. Supposing, then, we have a table exhibiting the elasticity or tension correspoc-iding to every degree of the thermometer, the weight of a given volume " vap as follows Steam at 21 2°, and under a pre ure of 30 inches of mercury, is 1700 times lighter than an equal bulk of water at its greatest density, or a temperature of about 40°, and a cubic foot of water at that temperature weighs 437272 grains; the weight, therefore, of a cubic foot of steam at that temperature and pre ure is 437272 -f- 1700 = 2.')7-218 grains. Hence we may find the weight of an equal bulk of vapour of the same temperature under any othf.r given pre ure, suppose 0-5G of an inch; for the density being directly as the pre ure, we have 30 in.: C-5G in.:: 257-218 grs.: 4-801 grs., which is the weight required. 577 Having found the weight of a cubic foot of vapoui under a pre ure of 0-5G of an inch, and at the temperature 212-', we may find its weight under the same pre uie at any other temperature, suppose 60°. It is ascertained by experiment that all aeriform bodies, whether vapours or gases, expand the l-480th part of their volume for every acce ion of temperature equivalent to one degree of Fahrenheit's scale; therefore, reckoning a volume of gas at 32° as unity, its volume at 60° is to its volume at 212° as ^ "*" 480 '^ ''^ ' "*" 480 ' ^'^ *^ ^'^^^ ' ^'^^^ ' '"^^ ^^® **^"" sity and weight being inversely as the volume, we have 1-058: 1-375:: 4-801 grs.: 6-222 grs. for the weight of a cubic foot of vapour at temperature 60°, and under a pre ure of 0-56 of an inch of the mercurial column. The following table, abridged from DanielVs Meteorological E ays, shows the force or tension, weight, and expansion of aqueous vapour, at different temperatures oa Fahrenheit's scale: — Temp. Force. Weight of aCubic Foot.! Expansion. 0 ■ocs •856 •9334 5 •OS.'? 1-034 -91.38 '• 10 -o;)8 1-208 •9542 1.5 •119 1^451 •9616 20 •110 1-6S8 •9750 25 •170 2028 •9855 ' 50 •200 2-361 •9959 35 •210 2-.S05 1-0062 40 •280 3-2.39 1-0166 4.5 •340 5:8!)3 10270 60 •400 4-535 1-0375 55 •476 ^•312 1-0479 60 •560 6-222 1-0583 65 •657 7:2.30 1^0687 70 •770 8-392 1^0791 75 •906 9-7S0 1-0895 SO 1060 1 1'-333 11000 85 1-255 15 081 M104 90 l^.W 15-005 1-1208 95 1-636 17*009 1-1312 212 30000 257-218 1-3749 Having thus explained the principle of the condensation hygrometer, we will now describe one or two of the forms under which it has been most frequently constructed. Daniell's hygrometer is represented in the annexed figure, a and b are two thio gla balls of 1^ inch dia" " meter, connected together by a tube having a bore about l-8th of an inch. The tube is bent at right angles over the two balls, and the arm b c contains a small thermometer de, whose bulb, which should be of a lengthened form, descends into the bsiU b. This ball having been about two thirds filled with ether, is heated over a lamp till the fluid boils, and tho vapour i ues from the capillary tube / which terminates the ball a. The vapour having expelled the air from both balls, the capillary tube is hermetically closed by the flame of alamp. The other ball a is now to be covered with a piece of muslin. The stand gh is of bra , and the transver.se socket i is made to hold the gla tube in the manner of a spring, allowing it to turn and be taken out with little difficulty. A small thermometer A- Ms inserted into the pillar of the stand. The manner of using the instrument is this: — After having driven out all the ether into the ball b by the heat of the hand, it is to be placed at an open window, or out of doors, with the ball b so situated that the surface of the liquid may be on a level with the eye of the observer. A little ether is then to be dropped on the covered ball. Evaporation immediately takes place, which, producing cold upon the ball a, causes a rapid and continuous condensation of the ethereal vapour in the interior of the instrument. The consequent evaporation from the included ether produces a depre ion of temperature in the ball b, the degree of which is measured by the thermometer de. This action is almost instantaneous, and the thermometer begins to fail in two seconds after the ether has been dropjjed. A depre ion of 30 or 40 degrees is easily produced, and the ether is sometimes observed to boil and the thermometer to be Pp [s. 590]