month of June to 2.62 inches. If we suppose this to bear the same proportion to the whole year that the evaporation in Dr. Dobson's experiments for June do to the annual evaporation, we shall obtain an annual evaporation amounting to about 22 inches. This is much smaller than that obtained by Mr. Williams. But Dr. Watson's method was not susceptible of precision. He collected the vapour raised on the inside of a drinking-glass; but it was impossible that the glass could condense much more than one half of what did rise, or would have been raised, in other cir cumstances. But to counterbalance this, the experiments were made in the hottest part of the day, when much more vapour is raised than during any other part of it.

The most exact set of experiments on the evaporation from earth was made by Mr. Dalton and Mr. Hoyle during 1796 and the two succeeding years. The method which they adopted was this. Having got a cylindrical vessel of tinned iron, 10 inches in diameter, and three feet deep, there were inserted into it two pipes turned downwards for the water to run off into bottles: The one pipe was near the bottom of the vessel; the other was an inch from the top. The vessel was filled up for a few inches with gravel and sand, and all the rest with good fresh soil. It was then put into a hole in the ground, and the space around filled up with earth, except on one side, for the convenience of putting bottles to the two pipes; then some water was poured on to sodden the earth, and as much of it as would was suffered to run through without notice, by which the earth might be considered as saturated with water. For some weeks the soil was kept above the level of the upper pipe, but latterly it was constantly a little below it, which precluded any water running off through it. For the first year the soil at top was bare; but for the two last years it was covered with grass the same as any green field. Things being thus circumstanced, a regular register was kept of the quantity of rain-water that ran off from the surface of the earth through the upper pipe (whilst that took place), and also of the quantity of that which sunk down through the three feet of earth, and ran ou through the lower pipe. A rain-guage of the same diameter was kept close by to find the quantity of rain for any corresponding time. The weight of the water which ran through the pipes being subtracted from the water in the rain-guage, the remainder was

considered as the weight of the water evaporated from the earth in the vessel. The following Table exhibits the mean annual result of these experiments *.

6.877-10-934- 7.379 8.402 33.560 25:158

Rain.. 30.629— 38·791— 31.259

[Evap.. 23.725— 27·857— 23·862

From these experiments it appears, that the quantity of vapour raised annually at Manchester is about 25 inches; if to this we add five inches for the dew, with Mr. Dalton, it will make the annual evaporation 30 inches. Now if we consider the situation of England, and the greater quantity of vapour raised from water, it will not surely be considered as too great an allowance if we estimate the mean annual evaporation over the whole surface of the globe at 35 inches. Now 35 inches from every square inch on the superficies of the globe make 94,450 cubic miles, equal to the water annually evaporated over the whole globe.

Were this prodigious mass of water all to subsist in the atmo. sphere at once, it would increase its mass by about a twelfth, and raise the barometer nearly three inches. But this never happens; no day passes without rain in some part of the earth; so that part of the evaporated water is constantly precipitated again. Indeed it would be impossible for the whole of the evaporated water to subsist in the atmosphere at once, at least in the state of vapour.

→ Manchester Memoirs, v. p. 360.

The higher regions of the atmosphere contain less vapour than the strata near the surface of the earth. This was observed both by Saussure and Deluc, who mentions several striking proofs of it.

At some height above the tops of mountains the atmosphere is probably still drier; for it was observed both by Saussure and Deluc, that on the tops of mountains the moisture of the air was rather less during the night than the day. And there can be little doubt that every stratum of air descends a little lower during the night than it was during the day, owing to the cooling and condensing of the stratum nearest the earth. Vapours, however, must ascend very high, for we see clouds forming far above the tops of the highest mountains.

[Thomson's Chem.]

The exact cause of evaporation, however, is still doubtful. The chief theories are those of vesicular vapour; solution in air; electricity. Richman, in the St. Petersburgh Memoirs, thinks the evaporation nearly in proportion to the temperature. Franklin conceives that evaporation, properly so called, is a solution of water in air, but that water or dust may be supported in the air by adhesion. Phil. Trans. 1765. Desaguliers believes, that vapour may be raised by electric attraction in the air. He makes the specific gravity of steam 400, from observations by Beighton and himself; or 336 from Nieuwentyt's experiments on the eolipile and hence infers, that vapour in summer heat should be about as dense as water, and should therefore float in air. But from his own experiments the specific gravity should be about five times as great.-Phil. Trans. Rep. of Arts.

Eeles, Phil. Trans. 1775, contends against the existence of vesi cular vapour, in favour of electrical atmospheres. Monge espoused the same side of the question as did Eason, Manch. Mem. i. 395. Darwin, in remarking on Eeles's opinions, Phil. Trans. 1757, supposes that the particles of vapour are real steam, but incapable of communicating their heat, perhaps on account of some motion. Hamilton objects both to vesicles and to fixed fire, and maintains the doctrine of solution in air. Phil. Trans. 1765.

Deluc, Phil. Trans. 1792, maintains that vapour exists in air precisely as in a vacuum, the distance at which its particles can remain without uniting with each other being determined only by

the temperature, and not being affected by the interposition of air. Wernon, in Mem. Goth. vi. 1. opposes Deluc's theory: and both has ascertained, by many experiments, that the pressure of air is indifferent to the quantity of vapour.

Professor Parrot considers the moisture contained in air as existing in two distinct states, of chemical and of physical vapour: he thinks the chemical vapour is sustained merely by the oxygen gas contained in the air, and that it is precipitated in consequence of the diminution of the oxygen; and the physical vapour he supposes to be merely interposed between the interstices of the elastic particles of air, and retained in its situation by heat: that the chemical solution of water or ice resembles oxidation, but that no physical evaporation can take place under the freezing point. Mr. Parrot builds his theory principally on eudiometrical experiments with phosphorus, which are attended with a copious precipitation, while the absorption of oxygen seems also to be much accelerated by the presence of water; but these experiments do not appear to be, by any means, decisive in favour of Mr. Parrot's theory. The same paper contains a proposal for inoculating the clouds with thunder and lightning, by projecting a bomb to a suffi. cient height.

Halley calculated that the evaporation of the Mediterranean in a summer's day is 5280 million tuns, and that the nine principal rivers furnish only 1827 millions. But the experiment on evapo. ration was made on a surface too small for the comparison.

The quantity of invisible moisture, contained in air, may be, in some degree, estimated from the indications of hygrometers, although these instruments have hitherto remained in a state of great imperfection. A sponge, a quantity of caustic potash, or of sulphuric acid, or a stone of a peculiar nature, has sometimes been employed for determining the degree of moisture of the air, from which it acquires a certain augmentation of its weight. A cord dipped in brine, or the beard of an oat, is also often used for the same purpose: the degree in which it untwists, from the effect of moisture, being shown by an index. But the extension of a hair, or of a slip of whalebone, which have been employed by Saussure and Deluc, appear to be more certain and accurate in their indications. The air hygrometer acquires more speedily the degree corresponding to any given state of the air, but it seems to

reach the utmost extent of its scale before it arrives at perfect humidity; while the whalebone hygrometer appears to express a greater change upon immersion in water than from the effect of the moistest transparent air, which has also been considered by some as an imperfection. Both these instruments are impaired by time, and acquire contrary errors, so that a mean between both is more likely to be correct than either separately. Their indications are at all times widely different from each other, and the mean appears to approach much nearer to a natural scale than either of them.

CHAP. XXXIX.

EDITOR.

FORMATION AND NATURE OF DEW, MIST, FOGS,
CLOUDS, RAIN, SNOW, AND HAIL.

SECTION 1.

General Remarks.

WHEN visible vapour has been deposited from transparent air, by

means either of cold or of some other cause, it generally remains for some time suspended, in the form of a mist or of a cloud: some. times, however, it appears to be at once deposited on the surface of a solid, in the form of dew or of hoar frost; for it is not probable that the chrystallized form, in which hoar frost is arranged, can be derived from the union of the particles already existing in the air as distinct aggregates.

The dew, which is commonly deposited on vegetables, is partly derived, in the evening, from the vapours ascending from the heated earth, since it is then found on the internal surface of a bell glass; and towards the morning, from the moisture descending from the air above, as it begins to cool. Sometimes, however, in warmer weather, the dew begins to descend in the evening;