An account of the circulation of the watry vapours of the sea, and of the cause of springs
An Account of the Circulation of the watry Vapours of the Sea, and of the Cause of Springs, presented to the Royal Society. By E. Halley.
Some tine since, I shewed 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 Command 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 a sufficiently assured that the Sea has not sensibly decreas’d by the loss in Vapour; nor yet abounded by the immense quantity of fresh water it receives continually from the Rivers. To demonstrate this Equilibre of receipt and Expense 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 hear 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, be shewing that is an Atom of Water were expanded into a Shell or Bubble so as to be ten times as big in Diamiter as when it was Water, such an Atom would become specifically lighter than Air, and rise so 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; and that warmth declining, and the Air growing cooler and the withal specifically lighter, the Vapours consequently shall stop at a certain Region of the Air, or else descend, which may happen upon several Accounts as I shall by and by endeavour to make out. Yet I undertake not that, this is the only principal of the rise of Vapours, and there ,may not be a certain sort of matter whose canatus 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 is the true cause, it is in fact certain, that warmth does separate the particles of Water and emit them with a greater and greater Velocity as the heat is more and more intense, as is evident in the steam of a boiling Cauldron, wherein likewise the Velocity of the Assent of the Vapours does visibly decrease till they disappear, being dispersed into and assimulated with the Ambient Air. Vapours being thus raised by warmth, let us for a first supposition, that the whole surface of the Globe were all Water very deep or rather that the whole Body of the Earth were water, and that the Sun had his Diurnal course about it : I take it, that it would follow that the Air of it self would imbibe a certain quantity of Aqueous Vapours and retain them like Salts dissolved in Water; that the Sun warming the Air and raising a more plentiful Vapour from the Water in the day time, the Air would sustain a greater proportion of Vapour, 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 believed that in such case there would be any diversity of Weather, other than periodically, every year alike; the mixture of all Terrestrious, Saline, Heterogeneous Vapours being taken away: which as the are variously compounded and brought by the Winds seem to be the causes of those various Seasons which we now find. In this case the Airy Regions every where at the same height would be equally replenished with the proportion of Water it could contain, regard being only to be had to the different degree of the 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 doth from the Equator; as is observed in the Ocean between the Tropicks.
Next, Let is suppose this Ocean interspersed with wide spacious Tracts of Land, with high ridges of Mountains such as the Pyrenean, the Alps, the Apennine, the Carpathian in Europe; Taurus, Caucasus, Imaus and several others in Asia; Atlas and the Montes Lunae, with other unknown Ridges in Afirca, whence came the Nile, the Nigre, 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 rarifed as to retain but a small part of those Vapours that shall be brought thither by the Winds. Those Vapours therefore that are raised copiously in the Sea, and by the Winds are carried over the low Land in 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 Crannies of the stone; and part of the Vapour entering into the Caverns of the Hills, the Water thereof gathers as in an Alembick into the Basons of stone it finds, which being once filled, all the overplus of Water that comes thither runs over by t5he lowest place, and breaking out of the sides of the Hills, forms single Springs. Many of these running down by the Valleys or Gutts between the ridges of the Hills, and coming to unite, form little Rivulets or Brooks: Many of these again meeting in one common Valley and gaining the plain Ground, being grown united in one common Channel make such streams as the Rhine, the Rhone, 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 sum of all those Springs which break out on the South side of the Carpathian Mountains, and on the North side of the immense Ridg of the Alps, which is one continued Chain of Mountains from Switzerland to the Black Sea. And it may almost pass for a Rule, that the Magnitude of a River, or the Quantity of Water it Evacuates is proportionable to the length and height of the Ridges from whence its Fountains arise. Now this Theory of Springs is not a bare Hypothesis but founded on Experience, which is was my luck to gain in my abode at Saint Helena, where in the Night time, on the tops of the Hills about 800 yards above the Sea, there was so strange a condensation, or rather precipitation of the Vapours, that is was a great Impediment to my Celestial Observations; for in the clear Sky the Dew would fall so fast as to cover each galf quarter of an hour, my Glasses with little drops,, so that I was necessitated to wipe them so often, and my Paper on which I wrote my Observations would immediately be so wet with the Dew, that is would not bear Ink: by which it may be supposed how fast the Water gathers in those mighty high Ridges I but now named.
Thus is one part of the Vapours blown upon the Land and 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 greater part of the whole Vapour, because of the 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 Mediterrenean as is extracted in Vapour. A third part falls in 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 come to pass, 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, does by the Valleys or lower Parts of the Earth find its way into the Rivers, and so I 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 that derives them from a Filtration of Percolation of the Sea-waters through certain imaginary Tubes or Passages within the Earth, wherein they lose their saltness. This besides many other labouring under this principle 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 Vapour. This, is 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 Alembicks 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. If the difference between Rain and Dew, and the cause why sometimes ‘tis Cloudy, at other times Serene, be enquired, I can offer nothing like a proper solution thereof, only with Submission to propose Conjectures which are the best I can find, viz. That the Air being heap’d up by the meeting of two contrary Winds, when the Mercury is high, the Vapours are the better sustained and kept from Coagulating or Condensing into Drops, whereby Clouds are not so easily generated: and in the Night the Vapours fall down single as they arose in the imperceptible Atoms of Water. Whereas when the Mercury is low and the Air rarified by the exhaustion thereof, by two contrary Winds blowing from the Place; the Atoms of Air keep the Vapours not so well separated, and the coalese into visible drops in the Clouds; and from thence are easily drawn into greater drops of Rain. To which ‘tis possible and not improbably, that some sort of Saline or Angular Particles of Terrestrial Vapour being immixt with the Aqueous, which I take to be Bubbles, may but or break their Skins or Coats, and so contribute to their more speedy Condensation into Rain.