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An Estimate of the Quantity of Vapour raised out of the Sea
by the Warmth of the Sun. By E. HALLEY. -- [1687.]
We took a pan of water, about four inches deep, and seven % inches diameter, in which was placed a thermometer, and by means of a pan of coals, we brought the water to the same degree of heat, which is observed to be that of the air in our hottest summers ; the thermometer nicely showing it. This done, we affixed the pan of water, with the thermometer in it, to one end of the beam of the scales, and exactly counterpoised it with weights in the other scale, and by the application or removal of the pan
of coals, we found it very easy to maintain the water in the same degree of heat precisely. Doing thus, we found the weight of the water sensibly to decrease ; and at the end of two hours we observed that there wanted half an ounce troy, all but seven grains, or 233 grains of water, which in that time had gone off in vapour ; though one could hardly perceive it smoke, and the water not sensibly warm. This quantity in so short a time seemed very considerable, being little less than six ounces in 24 hours, from so small a surface as a circle of eight inches diameter.
If, therefore, water as warm as the air in summer, exhales the thickness of a 60th part of an inch in two hours from its whole surface, in 12 hours it will exhale they of an inch ; which quantity will be found abundantly sufficient to serve for all the rains, springs, and dews, and account for the Caspian seas being always at a stand, neither wasting nor overflowing ; likewise for the current said to set always in at the Straits of Gibraltar, though those Mediterranean seas receive so many and so considerable rivers.
Upon this supposition, every 10 square inches of the surface of the water yields in vapour per diem a cubic inch of water; and each square foot half a wine pint; every space of four feet square, a gallon; a mile square, 6914 tons; a square degree, supposed of 69 English miles, will evaporate 33 millions of tons; and if the Mediterranean be estimated at 40 degrees long and four broad, allowances being made for the places where it is broader by those where it is narrower (and I am sure I guess at the least), there will be 160 square degrees of sea ; and, consequently, the whole Mediterranean must lose in vapour, in a summer's day, at least 5280 millions of tons. And this quantity of vapour, though very great, is as little as can be concluded from the experiment produced. And yet there remains another cause, which cannot be reduced to rule, I mean the winds, whereby the surface of the water is licked up sometimes faster than it exhales by the heat of the sun ; as is well known to those who have con. sidered those drying winds which blow sometimes.
The Mediterranean receives these considerable rivers, the Iberus, the Rhone, the Tiber, the Po, the Danube, the Neister, the Borysthenes, the Tanais, and the Nile, all the rest being of no great note, and their quantity of water inconsiderable. These nine rivers, we will suppose each of them to bring down 10 times as much water as the river Thames, not that any of them is so great in reality, but to comprehend with them all the small rivulets that fall into the sea, which otherwise I know not how to allow for.
To calculate the water of the Thames, I assume that at Kingston bridge, where the flood never reaches, and the water always runs down, the breadth of the channel is 100 yards, and its depth three, it being reduced to an equality ; in both which suppositions I am sure I take with the most. Hence the profile of the water in this place is 300 square yards ; this multiplied by 48 miles, which I allow the water to run in 24 hours, at two miles an hour, or 84,480 yards, gives 25,344,000 cubic yards of water, to be evacuated every day; that is, 20,300,000 tons per diem; and I doubt not but in the excess of my measures of the channel of the river, I have made more than sufficient allowance for the waters of the Brent, the Wandel, the Lea, and Darwent, which are all that are worth notice, that fall into the Thames below Kingston.
Now if each of the aforesaid nine rivers yield 10 times as much water as the Thames does, it will follow that each of them yields but 20,300,000 of tons per diem, and the whole nine but 1,827,000,000 of tons in a day; which is but little more than 4 of which is proved to be raised in vapour out of the Mediterranean in 12 hours' time.
On the Circulation of the Watry Vapours of the Sea, and the
Origin of Springs. By Mr. E. HALLEY. - [1690–1.] Some time since I showed an experiment of the quantity of water raised in vapour from the surface of the sea in a day's time, which was so far approved by some honourable members of this Society, that I have received their commands to prosecute those enquiries, and particularly in relation to the method used by nature to return the said vapours again into the sea; which is so justly performed, that in many hundreds of years we are sufficiently assured that the sea has not sensibly decreased by the loss in vapour, nor yet abounded by the immense quantity of fresh water it receives continually from the rivers. To demonstrate this equilibrium of receipt and expence in the whole sea, is a task too hard for me to undertake, yet in obedience to those whom I have the honour to serve I shall here offer, what to me has hitherto seemed the most satisfactory account of this grand phenomenon. I have formerly attempted to explain the manner of the rising of vapour by warmth, by showing that if an atom of water were expanded into a shell or bubble, so as to be ten times as large in diameter as when it was water, such an atom would become specifically lighter than air, and rise as long as that flatus, or warm spirit, that first separated it from the mass of water, shall continue to distend it to the same degree ; but that warmth decreasing, and the air growing cooler, and so specifically lighter, the vapours consequently will stop at a certain region of the air, or else descend; which may happen on several accounts, as will appear below. Yet I assert not that this is the only principle of the rise of vapours, and that there may not be a certain kind of matter whose conatus may be contrary to that of gravity; as is evident in vegetation, wherein the tendency of the sprouts is directly upwards, or against the perpendicular. But whatever be the true cause, it is in fact certain that warmth does separate the particles of water, and emit them with a greater velocity as the heat is more intense, as is evident in the steam of a boiling cauldron, wherein likewise the velocity of the ascent of the vapours visibly decreases till they disappear, being dispersed into and assimilated with the ambient air. Vapours being thus raised by warmth, let us, in the first place, suppose, that the whole surface of the globe were all water to a great depth, or rather, that the whole body of the earth were water, and that the sun had its diurnal course about it. It would follow, that the air of itself would imbibe a certain quantity of aqueous vapours, and retain them like salts dissolved in water; that the sun warming the air, and raising more plentiful vapours from the water in the day-time, the air would sustain a greater proportion of them, as warm water will hold more dissolved salts, which, upon the absence of the sun in the nights, would be all again discharged in dews, analogous to the precipitation of salts on the cooling of the liquors; nor is it to be thought that in such case there would be any diversity of weather, other than periodically, every year alike; the mixture of all terrestrial, saline, heterogeneous vapours being taken away; which, as they are variously compounded, and brought by the winds, seems to be the causes of those various seasons which we now find. In this case the region of air, every where at the same height, would be equally replenished with the proportion of water it could contain, regard being only to be liad to the different degree of warmth, from the nearness or distance of the sun ; and an eternal east wind would blow all round the globe, inclining only to the same side of the east, as the latitude does from the equator, as is observed in the ocean between the tropics.
The Mediterranean is interspersed with wide and spacious tracts of land, with high ridges of mountains, as the Pyrenean, the Alps, the Apennine, the Carpathian in Europe; Taurus, Caucasus, Imaus, and several others in Asia; Atlas, and the Montes Lunæ, with other unknown ridges in Africa, whence come the Nile, the Niger, and the Zaire ; and in America the Andes, and the Apalatean mountains, each of which far surpass the usual height to which the aqueous vapours of themselves ascend, and on the tops of which the air is so cold and rarefied, as to retain but a small part of those vapours brought thither by the winds. Those vapours, therefore, that are raised copiously in the sea, and by the winds carried over the low lands to those ridges of mountains, are there compelled by the stream of the air to mount up with it to the tops of the mountains, where the water presently precipitates, gleeting down by the crevices of the stone, and part of the vapour entering into the caverns of the hills, they are collected, as in an alembic, into the basins of stone they find there, which being once filled, all the overplus of water that comes thither runs over by the lowest place, and breaking out by the sides of the hills forms single springs ; many of these running down by the valleys or guts between the ridges of hills, and coming to unite, form little rivulets or brooks; many of these again meeting in one common valley, and gaining the plains, being grown less rapid, they become a river; and many of these being united in one common channel, make such streams as the Rhine, the Rhone, and the Danube ; which latter one would hardly think the collection of water condensed out of vapour, unless we consider how vast a tract of ground that river drains, and that it is the aggregate of all those springs which break out on the south side of the Carpathian mountains, and on the north side of the immense ridge of the Alps, which is one continued chain of mountains from Switzerland to the Black Sea. And it may generally pass for a rule, that the magnitude of a river, or the quantity of water it discharges, is proportionable to the length and height of the ridges, from whence its fountains arise.
Thus, then, is one part of the vapours, blown upon the land, returned by the rivers into the sea, from whence they came. Another part, by the cool of the night, falls in dews, or else in rains, again into the sea, before it reaches the land, which is by much the greatest part of the whole vapour, because of the great extent of the ocean, which the motion of the winds does not traverse in a very long space of time. And this is the reason why the rivers do not return so much into the Mediterranean as is extracted in vapour. A third part falls on the lower lands, and is the pabulum of plants, where yet it does not rest, but is again exhaled in vapour by the action of the sun, and is either carried by the winds to the sea, to fall in rain or dew there, or else to the mountains, to be there turned into springs; and though this does not immediately happen, yet after several vicissitudes, of rising in vapour and falling in rain or dews, each particle of the water is at length returned to the sea from whence it came. Add to this, that the rain waters, after the earth is fully sated with moisture, by the valleys or lower parts of the earth finds its way into the rivers, and so is compendiously sent back to the sea.
After this manner is the circulation performed, and I doubt not but this hypothesis is more reasonable than that of those who derive all springs from the rain waters, which yet are perpetual and without diminution, even when no rain falls for a long space of time; or than that which derives them from a filtration of the sea waters through certain imaginary tubes or passages within the earth, wherein they lose their saltness. This latter hypothesis, besides many others, labours under this principal absurdity, that the greatest rivers have their most copious fountains farthest from the sea, and whither so great quantities of fresh water cannot reasonably be derived any other way than in vapour. This, if we may allow final causes, seems to be the design of the hills, that their ridges being placed through the midst of the continents, might serve as it were for alembics to distil fresh water for the use of man and beast, and their heights to give a descent to those streams to run gently, like so many veins of the macrocosm, to be the more beneficial to the creation.
Now this theory of springs is not a bare hypothesis, but founded on experience, which it was my luck to gain in my stay at St. Helena, where in the night-time, on the tops of