the purpose of equipoising the instrument, that it may remain upright in the liquid under trial.

I must now take leave of you; but there remain yet many observations to be made on fluids: we shall, therefore, resume this subject at our next interview.

CONVERSATION XI.

OF SPRINGS, FOUNTAINS, &c.

OF THE ASCENT OF VAPOUR AND THE FORMATION OF CLOUDS. OF THE PORMATION AND FALL OF RAIN, &c.—OF THE FORMATION OF SPRINGS.—OF

RIVERS AND LAKES.OF FOUNTAINS.

CAROLINE.

THERE is a question I am very desirous of asking you respecting fluids, Mrs. B., which has often perplexed me. What is the reason that the great quantity of rain which falls upon the earth and sinks into it, does not, in the course of time, injure its solidity? The sun and the wind, I know, dry the surface, but they have no effect on the interior parts, where there must be a prodigious accumulation of moisture.

Mrs. B. Do you not know that, in the course of time, all the water which sinks into the ground rises out of it again? It is the same water which successively forms seas, rivers, springs, clouds, rain, and sometimes hail, snow, and ice. If you will take the trouble of following it through these various changes, you will understand why the earth is not yet drowned by the quantity of water which has fallen upon it since its creation; and you will even be convinced, that it does not contain a single drop more water now, than it did at that period.

Let us consider how the clouds were originally formed. When the first rays of the sun warmed the surface

of the earth, the heat, by separating the particles of water, rendered them lighter than the air. This, you know, is the case with steam or vapour. What then ensues?

Caroline. When lighter than the air it will naturally rise; and now I recollect your telling us in a preceding lesson, that the heat of the sun transformed the particles of water into vapour, in consequence of which it ascended into the atmosphere, where it formed clouds.

Mrs. B. We have then already followed water through two of its transformations; from water it becomes vapour, and from vapour, clouds.

Emily. But since this watery vapour is lighter than the air, why does it not continue to rise; and why does it unite again to form clouds?

Mrs. B. Because the atmosphere diminishes in density, as it is more distant from the earth. The vapour, therefore, which the sun causes to exhale, not only from seas, rivers, and lakes, but likewise from the moisture on the land, rises till it reaches a region of air of its own specific gravity; and there, you know, it will remain stationary. By the frequent accession of fresh vapour it gradually accumulates, so as to form those large bodies of vapour, which we call clouds: and these, at length, becoming too heavy for the air to support, they fall to the ground.

Caroline. They do fall to the ground, certainly, when it rains; but, according to your theory, I should have imagined, that when the clouds became too heavy for the region of air in which they were situated to support them, they would descend till they reached a stratum of air of their own weight, and not fall to the earth; for as clouds are formed of vapour, they cannot be so heavy as the lowest regions of the atmosphere, otherwise the vapour would not have risen.

Mrs. B. If you examine the manner in which the clouds descend, it will obviate this objection. In falling, several of the watery particles come within the sphere of each other's attraction, and unite in the form of a drop of water. The vapour thus transformed into

a shower, is heavier than any part of the atmosphere, and consequently descends to the earth.

Caroline. How wonderfully curious!

Mrs. B. It is impossible to consider any part of nature attentively without being struck with admiration at the wisdom it displays; and I hope you will never contemplate these wonders without feeling your heart glow with admiration and gratitude towards their bounteous Author. Observe, that if the waters were never drawn out of the earth, all vegetation would be destroyed by the excess of moisture; if, on the other hand, the plants were not nourished and refreshed by occasional showers, the drought would be equally fatal to them. If the clouds constantly remained in a state of vapour, they might, as you remarked, descend into a heavier stratum of the atmosphere, but could never fall to the ground; or were the power of attraction more than sufficient to convert the vapour into drops, it would transform the cloud into a mass of water, which, instead of nourishing would destroy the produce of the earth.

Water then ascends in the form of vapour, and descends in that of rain, snow, or hail, all of which ultitimately become water. Some of this falls into the various bodies of water on the surface of the globe, the remainder upon the land. Of the latter, part reascends in the form of vapour, part is absorbed by the roots of vegetables, and part descends into the bowels of the earth, where it forms springs.

Emily. Is rain and spring-water then the same?

Mrs. B. Yes, originally. The only difference between rain and spring water, consists in the foreign par ticles which the latter meets with and dissolves in its passage through the various soil it traverses.

Caroline. Yet spring-water is more pleasant to the taste, appears more transparent, and, I should have supposed, would have been more pure than rain water.

Mrs. B. No; excepting distilled water, rain water is the most pure we can obtain; and it is its purity which renders it insipid, whilst the various salts and dif

ferent ingredients, dissolved in spring water, give it a species of flavour, without in any degree affecting its transparency: and the filtration it undergoes through gravel and sand in the bowels of the earth, cleanses it from all foreign matter which it has not the power of dissolving.

When rain falls on the surface of the earth, it continues making its way downwards through the pores and crevices in the ground. When several drops meet in their subterraneous passage, they unite and form a little rivulet; this, in its progress, meets with other rivulets of a similar description, and they pursue their course together in the bowels of the earth, till they are stopped by some substance which they cannot penetrate.

Caroline. But you said that water could penetrate even the pores of gold, and they cannot meet with a substance more dense?

Mrs. B. But water penetrates the pores of gold only when under a strong compressive force, as in the Florentine experiment; now in its passage towards the centre of the earth, it is acted upon by no other power than gravity, which is not sufficient to make it force its way even through a stratum of clay. This species of earth, though not remarkably dense, being of great tenacity, will not admit the particles of water to pass. When water encounters any substance of this nature, therefore, its progress is stopped, and the pressure of the accumulating waters forms a bed, or reservoir. This will be more clearly explained by fig. 9. plate XIII. which represents a section, or the interior of a hill or mountain. A, is a body of water such as I have described, which, when filled up as high as B, (by the continual accession of water it receives from the ducts or rivulets a, a, a, a,) finds a passage out of the cavity, and, impelled by gravity, it runs on, till it makes its way out of the ground at the side of the hill, and there forms a spring, C.

Caroline. Gravity impels downwards towards the