Page images
PDF
EPUB

the air; and that, when it is very much rarified, or the dew falls upon their threads, they descend to the ground, or fall upon bushes, in the manner I mentioned above. Yet there remains one difficulty, which I shall be glad to see resolved, and that is, where those millions of spiders are bred; whether they deposit their eggs on earth, or in water, or on trees, from whence they can mount to such a height in the air, to feed upon little flies, as Dr. Lister observes, that afford them such a glutinous matter for the formation of their webs, which have that sticking quality. Conjectures, in an affair

, of this nature, are by no means satisfactory, and I have met with no experimental observations upon their origin.

. I am of opinion likewise, that this phenomenon was not known to, or at least is not escribed by any of the Greek and Roman naturalists. I know of no name for it in either of those languages. And those, who derive Gossamer from Gossipium, are led into that mistake, I believe, from the similitude of the sound; one being the produce of a shrub, and the other the work of spiders. I rather take Gossamer to be of a British or Saxon original. I observe, indeed, that Mr. Dryden makes use of that word, in his translation of a passage in Virgil's first Georgic, v. 397; but I think he is manifestly mistaken in the thing. Virgil says,

Tenuia nec lane per cælum vellera ferri. Doubtless meaning thereby fine fleecy clouds, according to the concurrent opinion of the commentators upon

that place. This Mr. Dryden incautiously renders thus:

The filmy Gossamer now flits no more. That the Gossumer was not unknown in Chaucer's time, appears from the following lines, in his Squier's Tale :

a

а

As sore wondren some on cause of thonder,
On ebbe and floud, on Gossomer, and on mist,
And on all thing til that the cause is wist.

By which Chaucer seems to intimate, that some naturalists; in or before his time, bad assigned the cause of the Gossamer, as well as of thunder, and of the flux and reflux of the sea; but what they made that cause is a doubt.

The fine contexture and appearance of the Gossamer in the air is humourously described by Shakespeare, in his Romeo and Juliet, in these words:

a

A lover may bestride the Gossamour,
That idles in the wanton summer air,

And yet not fall; so light is vanity. Lastly, I have not observed that this curious phenomenon has been taken notice of by any of those writers, who have given us the natural history of our counties in English; which I the more wonder at, as Dr. Plot, and some others, are very circumstantial in articles of less curiosity, and perhaps of less service, because the country people have a notion, that it is injurious to their cattle, being licked up in their feeding in the lattermaths, which is a thing worthy of a further inquiry 1759, Aug.

WM. MASSEY

XXIX. On the Influx of Water into the Mediterranean, NAVIGATORS unanimously attest, that in the straits of Gibraltar, between Cape Trafalgar and Cape Spartel, a strong current carries the water of the Atlantic, or Spanish sea, into the Mediterranean. This current, which is not at all times equally strong, is perceived in the Mediterranean at the distance of 20 English miles from the Straits towards the coast of Malaga. Some assure us that they have observed it at the distance of 70 miles near Cape Gaeta.

The existence of this current is confirmed by the chart of the Strait, published in 1700, by M. d'Ablancourt, who oba serves, that the constancy of the current is such in the middle of the Strait, that the tides make no variation in it; but that towards the two sides the water follows the ordinary laws of the Aux and reflux in the 24 hours. This chart is the more to be depended upon, as it was drawn by order of the king of Portugal, from careful observations made by the most able and experienced engineers and mariners.

Hudson adds, in the Philosophical Transactions, that in the middle of the Strait, which is about 5 English miles over, the current is carried towards the Mediterranean with such rapidity, that it runs at the rate of two miles an hour, and is so deep, that the longest line of a ship of war cannot reach the bottom of it. Other relations inform us that the strength of this current will carry a ship into the Mediterranean against the wind, if it be not very high. A few years ago a celebrated admiral confirmed this fact by his own experience. But he found, at the same time, that the upper part of the water in the Strait was indeed always carried into the Mediterranean; but that the water at bottom had a directly opposite direction, and ran from the Mediterranean into the Atlantic.

As the Mediterranean has no other sensible issue, but by the Straits of Gibraltar, and that, instead of emptying its water by this issue, it, on the contrary, continually receives fresh supplies by it, an embarrassing problem arises. Either the Mediterranean runs off by some unknown passage; or the water it receives is carried off by some secret power in

Mr. Kuhn adheres to the first of these opinions, and, in his treatise of the origin of springs, endeavours to prove that the Mediterranean hath a subterraneous gulph, by which its redundant water is discharged. But this supposition is confuted by facts; since it would be impossible for the water to run in with the rapidity we have just inentioned, if the Atlantic were not higher than the Mediterranean. If the two seas were of equal height, and the water of equal gravity, no reason could be assigned for the invariable direction of the current, which, according to the laws of hydrostatics, demonstrates that the Atlantic is the highest, consequently no water can run out of the Mediterranean into other seas by subterraneous channels, even supposing there were such ; on the contrary, those seas would supply the Mediterranean till it should obtain the requisite height and gravity.

Nevertheless, not only the Atlantic discharges itself into this sea, but also many great rivers run into it, to which must be added the water which falls in rain : as, therefore, its water cannot have any subterraneous issue, nature must employ some other method. Some naturalists have thought evaporation sufficient; and this opinion hath gained great probability since Mariotte proved that all the rain that falls annually is not sufficient to cover the globe of the earth to the height of eighteen or twenty inches; whereas the annual evaporation is about thirty or thirty-two inches.

Supposing then that the rain which falls annually into the Mediterranean, bears the same proportion to that which evaporates as at Paris, this sea would lose annually ten or twelve inches of water more than it receives. But what is carried to it by the Atlantic Ocean and by rivers much exceeds that quantity. And if we should even carry the evaporation much further, it would not account for the influx in à satisfactory manner; for we may admit that the water of the Mediterranean, being in a warmer climate than that of Paris, suffers an evaporation of twelve or fourte en inches more, that is to say, the quantity evaporated exceeds the rain that falls by twenty-four inches. The length of this sea is about a thousand leagues, of twenty-five to the degree, and its mean breadth may be about an hundred of those leagues; so that we can determine pretty exactly its surface to be 100,000 square leagues. For the rivers, then, to rea pair the annual diminution occasioned by evaporation, they must furnish, besides what is supplied by rain, a surface of 100,000 square leagues, with water to the height of twentyfour inches. Now according to Mariotte, the river Seine, in France, furnishes annually water enough to cover 561 square leagues the height of twelve inches. Riccioli, in his Geography Reformed, says, that the quantity of water furnished by the Po is to that of the Seine as 26} to 1; so that it would cover annually, to the height of twelve inches, a surface of 14,586 square leagues; which is about the fourteenth part of the water required to repair the evaporation of the Mediterranean. It would then only remain that we

. should consider the other rivers which empty themselves into it, as amounting altogether to fourteen times as much as the Po. Now as Riccioli attributes to the Nile seventeen times more water than to the Po, the Nile alone would furnish five times more water than would be necessary to supply the decrease made by evaporation. Supposing therefore that Riccioli has made the quantity of water carried by rivers to this sea too much by one half, as Sedileau proves that he hath done, there will still remain enough to make up the deficiency by evaporation.

Let us suppose the breadth of the Straits of Gibraltar to be a league of 25 to the degree, and that the water runs one such league in one hour: instead of a bottomless depth, let us take a depth of 200 feet only; the Mediterranean will then receive annually by the Straits a quantity of water, of 3,723,000 square leagues, and 24 inches in height, which will raise it annually 747 feet. But as the velocity of the current is not at all times equal, and as it is only in the middle of the Straits that the water is constantly carried towards the Mediterranean, it being subject at each side to the flux and reflux; to which must be added, what we observed before, that the water beneath follows a contrary direction, and is carried towards the Atlantic; these circumstances will oblige us to make a considerable abatement in the quantity of water which runs through the Straits. However, we may venture to assert that the water which the Mediterranean receives annually by the Straits and by the Nile increases its height at least twenty feet.

li

VOL. II.

But if we farther add the great rivers, such as the Danube, the Don, the Dneiper, the Dnister, and several others, which fall into the Black sea, and flow through the Straits of Constantinople into the Mediterranean, as also that multitude of rivers, great and small, which run on all sides into the Mediterranean, it will be evident that the height which this sea receives annually by those means cannot be less than 30 feet. That evaporation should carry off all this water, seems impossible; for in that case it would be twenty-five times stronger than at Paris, which is not situated in a cold climate. A lake of between 40 and 50 feet in depth, without any issue, would not dry up probably in a year, even under the line. M. de Buffon has nevertheless asserted, that evaporation is sufficient to carry off the surplus water which the Mediterranean receives annually. It was the authority of this celebrated naturalist that engaged M. Waiz to examine the subject with more exactness.

For this end, he considers the manner in which salt is made in the Mediterranean by natural evaporation, by receiving the water on a smooth surface to the height of an inch and a half only. This water evaporates in 24 hours, in the hottest season in the year, provided no rain falls. Dr. Hoffman tells us, that a pound of the Mediterranean water contains two lots (a lot is the 32d part of a pound] of salt : but according to the Swedish Academician's own experiments, salt water doth not deposit its salt till the evaporation is carried so far that there remain only five lots of salt to thire teen lots of fresh water. According to this calculation, evaporation on the coasts of the Mediterranean, in the hottest days, should carry off from each pound of water in the 24

, hours, 24 luts of water, which makes two thirds of an inch and a half which the water had in depth at the beginning. In deep cavities the evaporation must be more slow. In this manner the evaporation would in 24 hours, be one ineh and a half. But if we grant that this inch and a half of water is entirely evaporated in 24 hours, the salt remaining quite dry, and inaking the 33d part of the whole mass; the daily evaporation will then amount to 11 of an inch, and the anmual evaporation to 4475 feet, if it be equally hot all the year, and no rain falls. But as the hot weather lasts for some months only, and there are few days without rain, andi as there are even some whole seasons in which it rains constantly in the Mediterranean, and the evaporation is less, we cannot make the evaporation amount annually to 44 feet, especially as Lemery assures us, in his Course of Chymistry, that at Rochelle, in 15 days in the most proper season, fromi

2

« PreviousContinue »