Buoyancy

“ Only in rare cases does prolonged drying increase the period of flotation by more than a few days, examples being given at the end of the Table of Buoyancy results under Note 10. Adherent air-bubbles, a common cause of adventitious buoyancy, must always be removed. ”

“ When immersed and inflated, they remove a quantity of water adequate to the air which they contain, and, if fixed to a ponderous body, give it a degree of buoyancy proportionate to the water they displace. ”

“ When this separating motion becomes weakened in the water by the cold, the particles of compressed air concealed in the water collect; and, becoming larger, they become more capable of acting outwards through their buoyancy. ”

“ When pushed forward into deep water, vast masses are lifted up by their inherent buoyancy, and, broken off at the landward end, are borne away by the winds, or on tides and currents, to parts of the sea far removed from their place of formation. ”

“ It is well to have some water in your neighborhood, to give buoyancy to and float the earth. ”

“ Did you not learn in our conversation of yesterday, that air has weight, and exerts a pressure as much upwards as downwards and laterally? Were this not the case, how could the air in the interior of our bodies counteract the pressure of the atmosphere? The form of the bubble proves the same fact in a different way; for, had the air in its cavity pressed more in any one direction than in another, the bubble could not have been round, or, to speak more correctly, a sphere. ”

“ Where do the vapors which form this cloud-ring, and which are here condensed and poured down into the sea as rain, come from? They come from the trade-wind regions (§ 115) ; under the cloud-ring they rise up; as they rise up, they expand; and as they expand, they grow cool, form clouds, and then are condensed into rains; moreover, it requires no mercurial instrument of human device to satisfy us that the air which brings the vapor for these clouds can not take it up and let it down at the same temperature. ”

“ Now, besides the mutual Affinity that Water has to almost all Kinds of Air, and to Floguiston; added to its Power of Absorption; and as the sea, particularly in Summer, also rivers and damp meadows are generally cooler than the Lands and Countries bordering on them; Currents of damp cool Air press forwards to supply the Defect or Vacancy caused by Heat, Rarefaction and Elevation of dry warm Air, which is necessarily, and almost constantly rising into the Atmosphere, from heated Lands, Plains, and gentle Eminences long shone on by the Sun. ”

“ Perhaps you may ask how it is that the vapor so universally present gets into the atmosphere, and where it comes from. If you pour a little water into a plate, and set it down in the open air, you will note in the course of a day or two, that the water has sensibly diminished. The air has drunk up part of it, and will drink up the whole, if the water is allowed to stand long enough. What takes place from a small quantity of water goes on from every surface of water on the face of the earth, from every brook and river and lake, and from the great sea itself. ”

“ Wherever the sun-waves touch the rivers, ponds, lakes, seas, or fields of ice and snow upon our earth, they carry off invisible water-vapour. They dart down through the top layers of the water, and shake the water-particles forcibly apart; and in this case the drops fly asunder more easily and before they are so hot, because they are not kept down by a great weight of water above, as in the kettle, but find plenty of room to spread themselves out in the gaps between the air-atoms of the atmosphere. ”

“ The thirteen inches of water are of course supplied by evaporation from the sea, and are carried back to the land through the atmosphere. Vapour is perpetually rising from the ocean, and is condensed in the hills and high lands, and through their pores and crevices descends, till it is deflected, collected, and conducted out to the bay, by some stratum or channel which is watertight. The condensation which takes place in the higher parts of the country, may easily be recognised in the mists and rains which are the frequent occupants of such regions. ”

“ Also of a low barometer.—But, be that as it may, the air which these vapour-bearing winds—vapour-bearing because they blow over such an immense tract of ocean—pour into this stopping-place has to ascend and flow off as an upper current, to make room for that which is continually flowing in below. In ascending it expands and grows cool, and, as it grows cool, condensation of its vapour commences; with this, vast quantities of latent heat, which converted the water out at sea into vapour for these winds, are set free in the upper air. ”

“ Thus the water, which by parting with its heat of liquefaction, has expended its meteorological energy in giving dynamical force to the air, is like the exhausted steam of the engine; it has exerted its power and become inert. ”

“ If, in the Middle of a hot sunny Day, Vapours lighter than the Air, were to rise from the Ocean, (which they will continue to do, in hollow Vesicles or Bladders, till the Expansion breaks the Bubble, at which Time the Water woud fall to the Earth, if not drank up by the Attraction of dry Spunges of Air;) there woud be a constant Wind blowing from Land to Sea, to fill up the Chasm: but at such Time, the Land is more heated than the Sea: therefore hot Air and Vapour arise from both; and the Breeze, on the contrary, blows from Sea to Land ”

“ For here the wind not only has free scope, but it also finds another element, fluid but incompressible, to obey its impulse and follow in the course which it takes. If the WIND be like a wild spirit, the WATER is a mighty, irresistible body, endowed with motion by the other, and capable of any work, from the drowning of a sea-gull to the wrecking of a stately ship. ”

“ Can you imagine these water-particles, just above any pond or lake, rising up and getting entangled among the air-atoms? They are very light, much lighter than the atmosphere; and so, when a great many of them are spread about in the air which lies just over the pond, they make it much lighter than the layer of air above, and so help it to rise, while the heavier layer of air comes down ready to take up more vapour. ”

“ Rain comes from the contraction of the clouds by cold. A cold current of air coming in contact with a cloud will condense its bubbles into drops, and these of course will fall. The same result occurs if a cloud passes into a cold stratum of air. But let us look at the process more minutely. Let us see what the effect of cold is upon the bubbles. The first effect may be made clear by Fig. 208. If a bubble be contracted by the influence of cold, the water of its wall being made thicker, there will be a gathering of it from gravitation at the lower part, as represented by the dotted line. ”

“ As the air rises, there is less air above it to press down on it; so it expands. When air expands it cools, and the water vapor which is mingled with it likewise cools. When the water vapor gets cool enough it condenses, changing to myriads of extremely small drops of water. These make a cloud.
A wind comes along; that is, the air in which the cloud is floating moves. The wind carries the cloud [pg 279] along with it. More rising air, full of evaporated water from the ocean, joins the cloud and cools, and the water forms into more tiny droplets. ”

“ We see water passing from solid ice to liquid water and gaseous steam, by a mere rise in temperature, and conversely, by abstraction of heat, steam passes back to water, and then to ice.
When one’s hands get wet they get cold. A commonplace, but convenient proof of this is to wet the finger in the mouth and hold it in the air. A sensible reduction of temperature is instantly noticeable. A more pronounced illustration is to wet the hands in a basin of water, and then plunge them in the blast of hot, dry air coming from a furnace register. ”

“ The sun and the air absorb or suck up the water and carry it off to their homes. Now, the bubbles are made of a little water and a little air. The water is on the end of the pipe, and Tommie blows the air into the pipe, and the bubble grows big and round. When it breaks, the air sucks up the water, which was the outside of the bubble, and the air which was inside mixes with the air in the room. ”

“ A stop is put to this new generation of air, when about a fifteenth of the whole is produced, by its increasing pressure; and a similar boundary is fixed to its absorption or destruction by the decrease of atmospheric pressure. As water requires more heat to convert it into vapour under a heavy atmosphere than under a light one, so in letting off the water from muddy fish-ponds great quantities of air-bubbles are seen to ascend from the bottom, which were previously confined there by the pressure of the water. ”

“ Now when we come to recollect that evaporation is lifting up, that the winds are transporting, and that the clouds are letting down every day actually such a body of water, we are reminded that it is done by little and little at a place, and by hairs' breadths at a time, not by parallelopipedons one mile thick, and that the evaporation is most rapid and the rains most copious, not always at the same place, but now here, now there. ”

“ It is not enough that we have the evaporation of moisture that will ascend into the higher regions of the air and there be condensed into cloud and possibly rain, but we must have the means for distributing these conditions over a large area, and for this purpose we have the phenomenon of wind. Why the winds blow can be accounted for to a certain extent,—we might say to a large extent,—but there yet remain many unsolved problems relating to wind and weather. Of the phenomena of wind we will speak more fully in a future chapter. ”

“ Not less than three-fourths of the vapour which makes this rain comes from this waste of waters; but supposing that only half of this quantity, i.e., ninety-three thousand one hundred and twenty cubic miles of rain falls upon this sea, and that that much, at least, is taken up from it again as vapour, this would give two hundred and fifty-five cubic miles as the quantity of water which is daily lifted up and poured back again into this expanse. ”

“ These springs feed the streams and rivers which carry the water back again to the sea. It is owing to this continuous circulation of water that the sea does not overflow, or the rivers dry up. Moreover, those damp exhalations supply the air with that moisture which is necessary to the life of men and beasts, and to the growth of plants. One thing more. You know that water turns foul when it remains for some time without being stirred. How is it that though it is shut in one place, the water of the sea never turns foul? ”

“ Here it is rendered latent by the forces of evaporation, and in this form, having fulfilled its office in the economy of the ocean, it passes off into the air, there to enter, in mysterious ways, upon the performance of its manifold tasks.
890. Actinic processes.—As evaporation goes on by day or night, the upper stratum is rendered heavier by reason of both the heat and the fresh water borne away by evaporation; the upper water having been thus rendered both Salter and cooler, has its specific gravity increased so much the more. ”

“ Experimental philosophy demonstrates, that water is incompressible, for instead of shrinking and entering into itself when pressed, it [85] passes through the most solid and thickest vessels; which could not be the case if the air it contained were in a state of full elasticity. The air contained therefore in water, is not simply mixed therewith, but is united in a state where its spring is not sensibly exercised; yet the spring is not entirely destroyed, for if we expose water to congelation, the air issues from its internal part, and unites on its surface in elastic bubbles. ”

“ The water in all these rivers, lakes, and oceans is constantly rising into the air in what is called moisture or vapor. We can not see this moisture, neither can we see the air.
If the air is cold, moisture does not rise rapidly; but, as the air becomes heated, it takes up more moisture, so that the more heat there is in the air, the more moisture rises. ”

“ Upward and downward movements in fluids are consequent upon each other, and they involve lateral movements. The sea, with its vapour, is the great engine which gives upward motion in the air. As soon as aqueous vapour is formed it rises; the air resists its ascent; but it is lighter than the air, therefore (§ 177) it forces the resisting particles of air up along with it, and so produces ascending columns in the atmosphere. The juxta air comes in to occupy the space which that carried up by the vapour leaves behind it, and so there is a wind produced. ”

“ For instance, a cold pitcher in the summer robs the vapor in the air of its heat, and causes it to be deposited on its own surface,—of course the water comes from the atmosphere, not through the wall of the pitcher; if we breathe on a knife blade, it condenses, in the same manner, the moisture of the breath, and becomes covered with a film of-water ”

“ Thus, between tide-waves and currents, changes are going on continually both in the volume and the physical constitution of the sea-water. And this, which might at first on a hasty glance look like casual and unimportant variation, when we come to consider it attentively, is found to be an arrangement fraught with wisdom and beneficence; for it insures continual variations of temperature, it attracts the purifying storms, restores the elastic spring of the atmosphere, and provides for weary man a healthy tonic breeze when he rambles along the beach or scales the face of the cliff. ”

“ As the specifick gravity of air impregnated with water is much less than pure air, a vertical motion, or rising up of this aqueous air, and a rushing in of more dense air to fill up the vacuum, will be continually going on; and thus a large quantity of heat abstracted from the adjacent atmosphere. This observation receives considerable confirmation from situations such as have been mentioned enjoying more uniform breezes than other situations less contiguous to the river. ”