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vapour which it contains. If the quantity be as great as can exist at the given temperature, then the smallest diminution of temperature will occasion the deposition of humidity. But if air at 72o contain only as much yapour as it can retain at the temperature of 52°, it is obvious that it must be cooled below 52° before it begin to deposit moisture.
During the day a good deal of water is converted into vapour from the surface of lakes, seas, and rivers, and from the earth itself, and mixes with the atmosphere. The temperature of the atmosphere usually sinks considerably after sunset, and is often 20° or 30° colder than at the hottest part of the day. Hence it must approach much nearer the point of depositing moisture than during the day. The greatest difference between the temperature of day and night takes place in this country in spring and autumn, and these are the seasons in which the most abundant dews are usally deposited. Dewy nights are usually clear. On cloudy nights dew seldom falls.
Many years ago, a curious set of experiments on dew was made by M. Dufay. He placed a glass cup in the middle of a silver basin, and left both in the open air during a dewy night. Next morning the silver basin was found dry; but the glass cup was wet with dew. When the experiment was reversed by placing a silver cup in the middle of a glass basin, the glass was still moist and the silver dry. These and many
other similar experiments, remained unexplained till Dr. Wells turned his attention to the subject. It is only necessary to say that the metals are bad radiators of heat, while glass is a good radiator. . Hence in a cloudless night the temperature of the glass exposed to the aspect of the sky will sink much lower than that of metals. . It will cool the air in its neighbourhood more, and of course dew will be deposited on it in preference. Dr. Wells found, as Mr. Six had done before him, that a thermometer laid on a grass plot in a clear night sunk 6°, 8°, 13°, or even 20° lower than a thermometer hung at some height from the ground. Because grass radiates heat well. In short, dew is deposited on those substances which radiate heat well, while it avoids, for an obvious reason, all bad radiators. These depositions do not take place on cloudy nights, because clouds radiate the heat back again, and thus prevent the temperature of good radiators from sinking much below that of the atmosphere.
• In frosty weather moisture is almost always condensed upon the inside of the windows of our apartments (during the night when the room is without fire), in the form of dew or hoar frost. The glass being a good radiator, is speedily cooled below the temperature of the room. Vapour from the air in the apartment is consequently condensed upon it, and it assumes the form of dew or hoar frost according to the temperature of the glass. This condensation is much more abundant when the window shutters are closed than when they are left open. Because in the latter case the radiation from the different parts of the room upon the window, supplies a considerable portion of the heat radiated by the glass, and prevents the temperature from sinking so low.'-pp. 165–168.
Under the head of “Fluidity,” the author treats of frigorific mixtures. It is pleasant to know that in the midst of summer, cold liquids may be produced sufficient for all domestic purposes, without the aid ot preserved ice or snow. The principle on which this artificial cold may be obtained, is the expeditious solution
of any salt which contains a great deal of water of crystallization, The common Glauber salts, (sulphate of soda), which may be had for a few pence a pound, is the best and the readiest salt we know of for the purpose of producing cold in summer. Suppose a person desires to cool a bottle of wine in summer to the temperature at which it should properly be drunk. Ice, we do not hesitate to say, is totally inadmissible. It is now exploded in the best companies, as being too cold, harsh, and too often actually decomposing the wine. A mixture that will reduce the temperature of wine fifty degrees below what it stands at in our cellars, ought to satisfy the coolest patron of the juice of the grape that ever gloried in a rubicund cheek. This mixture he can easily obtain by following this recipe. Take of Glauber salt two ounces, one drachm, and one scruple-(one pennyworth); of sulphuric acid, undiluted, which can be weighed in a bottle, one ounce and a scruple-(one pennyworth also); of water, which should also be weighed, five drachms, one scruple, and thirteen grains. These proportions should be accurately attended to, as the least variation in them will be attended with a great difference in the result. First mix the sulphuric acid and the water together, and since this process will cause a great deal of sensible heat, wait until the mixture becomes perfectly cool. When it is cool, then, and not until then, pound the salts so as to reduce it to a powder, which should be done as soon as possible, and then throw it into the mixture, which will soon become very cold. Such a preparation as this is the best that wine can be subjected to in order to free it from the ill effects of a badly constructed cellar.
The chapter on vaporization is full of interest; and the principle on which it is produced, the more deserves our attention, since it serves to explain natural phenomena which occur at certain seasons of the year before our eyes every day. Dr. Thomson's observations on this subject will be admitted to be curious and important.
Every body knows that water evaporates at all temperatures, however low. After a heavy fall of rain the roads become deep, and the country becomes studded with little ponds of water. But after a few days or weeks of fair weather, the roads get dry and dusty, and the little ponds of water disappear. And this takes place not only in summer but even in winter, when the weather happens to continue dry for some time. The Mediterranean sea receives many very large rivers. 'The Nile, the Po, the Rhone, the Ebro, the Danube, the Nieper, the Don, and many other rivers of smaller extent empty themselves either directly into the Mediterranean or into the seas connected with it, and constituting as it were a part of this great inland ocean. Yet notwithstanding this great and regular influx of
. water, this sea not only does not increase in size ; but a constant current sets in from the Atlantic through the Straits of Gibraltar. An evident proof that the natural evaporation from the surface of the Mediterranean, is more than sufficient to dissipate all the water thrown into it from a vast tract of Europe and Africa.'--p. 241.
But the importance of this spontaneous evaporation ought to be duly appreciated. Dr. Thomson says
* The property which water has of evaporating spontaneously at all temperatures, is one of the most important in the whole economy of nature. For upon it the growth of plants, and the existence of living creatures upon the earth, depends. The vapours thus continually rising, not merely from the surface of the sea, lakes, and rivers, but also from the dry land, are again condensed and fall in the state of rain or dew. The rain penetrates into the earth, and makes its way out again in springs. These collecting together constitute rivers, which making their way to the sea, afford the means of living and enjoyment to numerous tribes and languages which occupy their banks. Let us suppose for a moment that this spontaneous evaporation were to cease, and let us contemplate the consequences. No more rain or dew could fall, the springs would cease to flow, the rivers would be dried up; the whole water in the globe would be accumulated in the ocean ; the earth would become dry and parched ; vegetables being deprived of moisture, could no longer continue to grow; the cattle and beasts of every kind would lack their usual food ; man himself would perish; the earth would become a dull, inanimate, steril mass,
any vegetables to embellish its surface, or any living creature to wander through its frightful deserts.
• If the atmosphere contained no vapour whatever, the annual evaporation from the surface of water could easily be determined from the data already stated in this section, provided we were acquainted with the mean temperature of the place. But as the atmosphere is never free from vapour, we must either determine the mean quantity present by trial, or determine the actual evaporation by experiment. Now as far as evaporation is concerned, the surface of the globe presents three principal varieties; namely, water, ground covered with grass or other vegetables, and bare soil.
· Dr. Dobson made a set of experiments during the years 1772, 1773, 1774, and 1775, to determine the evaporation from the surface of water at Liverpool during these years. He took a cylindrical vessel of twelve inches diameter, and having nearly filled it with water, exposed it beside a rain gauge of the same aperture, and by adding water, or removing it occasionally, he kept the surface at nearly the same height. By carefully registering the quantities added or taken away, and comparing them with the rain that fell, the amount of evaporation was ascertained. The mean annual evaporation from the surface of water at Liverpool amounted to 36.78 inches. The mean annual fall of rain at Liverpool, as ascertained by Dr. Dobson, is (without reckoning the dew) 37.48 inches. We see at once from this that more rain falls at Liverpool than can be accounted for by the evaporation. Consequently there must be a supply of vapour from the sea, and probably from the varmer regions of the globe.
*A set of experiments upon the evaporation from ground covered with vegetables and from bare soil was made by Mr. Thomas Hoyle and Mr. Dalton, at Manchester, during the years 1796, 1797, 1798. They got a cylindrical vessel of tinned iron, ten inches in diameter, and three feet deep. There were inserted into it two pipes turned downwards for the water to run off from it into bottles. One of these pipes was near the bottom of the vessel, the other was an inch from the top. This vessel was filled up for a few inches with gravel and sand, and all the rest of it with
goud 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. Water was poured on to sadden the earth, and as much as would was suffered to run through without notice, by which the earth might be considered as saturated with water. weeks the soil was constantly above the level of the upper pipe, but latterly it was always a little below it,; which made it impossible for any water to run through the upper pipe. For the first year the soil at top was bare, but during the last two years it was covered with grass the same as a 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 by the upper pipe (while that took place,) and also of the quantity which sunk down through the three feet of earth, and ran out through the lower pipe. A rain gauge of the same diameter was kept close by to find the quantity of rain for any corresponding time. By this apparatus the quantity evaporated from the earth in the vessel during three years was ascertained. The annual evaporation was 25°158 inches. Now, if to the rain we add five inches for dew (not reckoned in Mr. Dalton's observations), it follows that the mean annual evaporation from earth at Manchester, amounts to thirty inches. It follows, likewise, from these observations of Dalton and Hoyle, that there is but little difference between the evaporation from green soil and bare soil. For the evaporation during the first year, when the soil in the vessel was bare, differed but little from that of the two following years when it was covered with grass.'--pp. 259–262.
Dr. Thomson now thinks it necessary to inquire how far the rain and the dew correspond with this evaporation. The mean fall of rain over all Great Britain, he estimates at less than thirty-six inches; the mean evaporation being assumed to be thirty-two inches; the remaining four inches, which are not elevated in the state of vapour, he calculates are yearly carried to the sea by rivers. the formation of rain, we have the following remarks:
• The formation of rain is still involved in impenetrable obscurity. Rain never falls in this country unless the sky be cloudy, and unless that peculiar kind of dense black cloud appear well known by the name of rain cloud. Whenever the particles constituting clouds lose their vesicular
. form and unite together in drops, rain falls. This change is probably connected with some electrical phenomena which are not yet understood. Clouds are attracted by mountains, and more rain falls in mountainous districts than in any other. We can conceive the mountain in the opposite electrical state from the cloud. This would account for the attraction. When the cloud came close to the mountain its electricity would be abstracted, and the vesicles in consequence might collapse into drops.
* In that part of Peru called Vallies, which lies on the north and south side of Lima, in south latitude 12° hounded on the east by the Andes, and on the west by the Pacific Ocean, it never rains at all.
But during winter the earth is covered with so thick a fog as to intercept the rays
of the sun. This fog appears almost every day during winter with a density that obscures objects at any distance. About ten or eleven o'clock it begins to rise, but without being totally dispersed; though it is then no impediment to the sight, intercepting only the direct rays of the sun by
day, and that of the stars by night. Sometimes it is so far dispersed that the disc of the sun becomes visible, but the heat from his rays is still precluded. In the winter season these vapours dissolve into a very small mist or dew, which they call garua, and thus every where moisten the earth.
These garuas never fall in any quantities sufficient to damage the roads or incommode the traveller; but they render the most arid and barren parts fertile. They convert the disagreeable dust in the streets of Lima into mud.
Now in that country the wind always blows from the south; that is from a colder to a warmer region. Sometimes it veers a point or two to the east. But it always blows between the south and south-east. When the fogs come on the south wind is barely felt, and a scarcely perceptible air seems to come from the north, which forms the fog.
* The obvious reason why it never rains in that country is, that the wind constantly blows from a colder to a hotter part of the world. We see also the cause of the fogs. They are occasioned by the mixture of the hot air from the north with the colder air from the south.
* Rain is produced by irregular winds. If the winds were always to blow steadily in the same direction no rain whatever would fall.
• When a country is quite flat, as is the case with Egypt, it seldom rains, although the winds are not quite steady. In Egypt it very seldom rains.
. During June, July, August, and part of September, the wind blows from the north. During the latter part of September it blows from the east. The winds are most variable about the winter solstice. From that to March they are mostly southerly.
. The heavy rains that fall in India always take place during the shifting of the monsoons, and while they last the winds are always variable. Even in this country steady dry weather is always accompanied by a steady direction of the wind, whereas in rainy weather the winds are unsteady and variable.
* These facts are sufficient to show the connexion of rain with the variable nature of the winds.'--pp. 275-277.
Under the head of Combustion, we have the annexed description of the nature of flame:
• Flame is the rapid combustion of volatilized matter. The tallow or the wax is melted and drawn up to the top of the wick of a candle. Here it is boiled and converted into vapour, which ascends in the form of a column. This
vapour is raised to such a temperature, that it combines rapidly with the oxygen of the surrounding atmosphere, and the heat evolved is such as to heat the vapour to whiteness. Flame then is merely volatile, combustible matter heated white hot. The combustion can only take place in that part of the column of hot vapour that is in contact with the atmosphere, namely, the exterior surface. The flame of a candle then is merely a thin film of white hot vapour, enclosing within a quantity of hot vapour which for want of oxygen, is incapable of burning. But as it advances upward in consequence of the outer film being already consumed, it gradually constitutes the outer surface of the column, and assumes the form of flame. And as the supply of hot vapour diminishes as it ascends, and at last fails altogether, the flame of a candle gradually tapers to a point. That this is the nature of flame has been beautifully shown by my late friend Mr. Oswald Sym, in a paper which has been greatly admired, but which