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always displaces a quantity of water equal to its weight.
Caroline. But you said just now, that in the immersion of sold, the bulk, and not the weight of body, was to be considered.
.>/;'>. //. That is the case with all substances which are heavier than water; but since those which are lighter do not displace so much as their own bulk, the quantity they displace is not a test of their specific giavity.
In order to obtain the specific gravity of a body which is lighter than water, you must attach to it a heavy one, whose specific gravity is known, and immerse them together; the specific gravity of the lighter body may then be easily calculated.
Emily. But are there not some bodies which have exactly the same specific gravity as water?
Mrs. B. Undoubtedly ; and such bodies will remain at rest in whatever situation they are placed in water. Here is a piece of wood which, by being impregnated with a little sand, is rendered precisely of the weight of an equal bulk of water ; in whatever part of this vessel of water you pUrp. it, you will find that it will remain stationary.
Caroline. I shall first put it at the bottom ; from thence, of course, it cannot rise, because it is not lighter than water. Now I shall place it in the middle of the vessel ; it neither rises nor sinks, because it is neither lighter nor heavier than the water. Now I will lay it on the surface of the water ; but there it sinks a little— what is the reason of that, Mrs. B.?
Mrs. B. Since it is not lighter than the water, it cannot float upon its surface; since it is not heavier
than water, it cannot sink below its surface: it will .sink therefore, only till the upper surface of both bodies are on a level, so that the piece of wood is just covered with water. If you poured a few drops of water into the vessel, (so gently as not to increase their momentum by giving them velocity) they would mix with the water at the surface, and not sink lower.
Caroline. This must, no doubt, be the reason why in drawing up a bucket of water out of a well, the bucket feels so much heavier when it rises above the surface of the water in the well ; for whilst you raise it in the water, the water within the bucket being of the same specific gravity as the water on the outside, will be wholly supported by the upward pressure of the water beneath the bucket, and consequently very little force will be required to raise it; but as soon as the bucket rises to the surface of the well you immediately perceive the increase of weight.
Emily. And how do you ascertain the specific gravity of fluids?
Mrs. B. By means of an instrument called an hydrometer, which I will show you. It consists of a thin glass ball A, (fig. 8, plate XIII.) with a graduated tube B, and the specific gravity of the liquid is estimated by the depth to which the instrument sinks in it. There is a smaller ball, C, attached to the instrument below, which contains a little mercury; but this is merely for 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.
OF SPRINGS, FOUNTAINS, &o.
Of the Ascent of Vapor and the Formation of Clouds.— Of the Formation and Fall of Rain, tifc.—Of the Formation of Springs.—Of Rivers and Lakes.—Of Fountains.
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 vapor. 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 vapor, in consequence of which it ascended into the atmosphere, where it formed clouds.
JUrs. B. We have then already followed water through two of its transformations; from water it becomes vapor, and from vapor clouds.
Emily. But since this watery vapor 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 vapor 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; aud there, you know, it will remain stationary. By the frequent accession of fresh vapor it gradually accumulates, so as to form those large