9. (Pg. 120) When is a fluid said to be in equilibrium?

10. (Pg. 120) What is there in the nature of a fluid, which causes it to seek this level?

11. (Pg. 120) What circ.u.mstances occasion oil to float upon water?

12. (Pg. 120) What is the nature and use of the instrument represented in fig. 1, plate 13?

13. (Pg. 120) What difference is there in the gravitation of solid ma.s.ses, and of fluids?

14. (Pg. 121) What results as regards the pressure of fluids?

15. (Pg. 121) How is this ill.u.s.trated by fig. 2, 3, plate 13?

16. (Pg. 121) From what does the lateral pressure proceed? and to what is it proportioned, as exemplified in fig. 5, plate 13?

17. (Pg. 122) Has the extent of the surface of a fluid, any effect upon its pressure downwards?

18. (Pg. 122) What will be the difference between the pressure upon the bottom, and upon one side of a cubical vessel?

19. (Pg. 122) What occasions the upward pressure, and how is it explained by fig. 4, plate 13?

20. (Pg. 123) How could the equilibrium of fluids be exemplified by pouring water in at the spout of a tea-pot?

21. (Pg. 123) How by the apparatus represented at fig. 6, plate 13?

22. (Pg. 123) What is meant by the specific gravity of a body?

23. (Pg. 123) What do we in common mean by calling a body heavy, or light?

24. (Pg. 124) Why would not the metals answer to compare other bodies with?

25. (Pg. 124) What must be supposed equal in estimating the specific gravity of a body?

26. (Pg. 124) What has been adopted as a standard for comparison?

27. (Pg. 125) What is the first step in ascertaining the specific gravity of a solid?

28. (Pg. 125) What quant.i.ty of water will the solid displace?

29. (Pg. 125) Why will a solid weigh less in water than in air, and to what will the loss of weight be equal?

30. (Pg. 126) What is the arrangement represented by fig. 7, plate 13?

31. (Pg. 126) What is stated of gold as an example?

32. (Pg. 126) In comparing a body with water, this is sometimes called 1000, what must be observed?

33. (Pg. 126) What quant.i.ty of water is displaced, by a body floating upon its surface?

34. (Pg. 127) How can you find the specific gravity of a solid which is lighter than water?

35. (Pg. 127) What is observed of a body whose specific gravity is the same as that of water?

36. (Pg. 127) What is the reason that in drawing a bucket of water from a well, its weight is not perceived until it rises above the surface?

37. (Pg. 128) Describe the instrument represented by fig. 8, plate 13, and also how, and for what it is used?

CONVERSATION XI.

OF SPRINGS, FOUNTAINS, &c.

OF THE ASCENT OF VAPOUR AND THE FORMATION OF CLOUDS. OF THE FORMATION 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 quant.i.ty 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 acc.u.mulation 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 quant.i.ty 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 acc.u.mulates, so as to form those large bodies of vapour, which we call clouds: and the particles, at length uniting, become too heavy for the air to support, and 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 grat.i.tude, 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 ma.s.s 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 ultimately 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 earth, where it forms springs.

_Emily._ Is there then no difference between rain water, and spring water?

_Mrs. B._ They are originally the same; but that portion of rain water which goes to supply springs, dissolves a number of foreign particles, which it meets with in its pa.s.sage through the various soils 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.