119 



SEA. 



SEA. 



410 



unless, indeed, the conjecture of Professor Schb'nbein, that it partly 

 arises from the oxidation of dead organic matter in the sea by means 

 of ozone, should hereafter be verified. The luminous animals belong 

 to every sub-kingdom of invertebrate anitnals : Protozoa ; Radiata, 

 especially the Acalepha: ; Crustacea, as a class of Annulosa ; Mollusca ; 

 and the osculent groups of the Annelida and .Tunicata; some of the 

 compound forms of the latter, or compound Ascidians, being among the 

 most splendidly phosphorescent. In the Transactions of the Berlin 

 Academy of Sciences is a valuable paper by Ehrenberg on the luminous 

 animals of the sea, containing a copious list of the species observed to 

 possess this property. [LUMINOSITY OF ORGANIC BEINGS, in NAT. 

 HIST. Div.] 



From the well-known laws of gravitation, it is inferred that the 

 surface of the sea is always at the same distance from the centre of 

 the earth, and that consequently it forms a uniformly regular curve. 

 This surface of course maintains the same level, and it is consequently 

 the best basis from which to determine the relative elevations of the 

 different parts of the land. Some facts bearing on this subject, and on 

 the precautions requisite in the use of the sea-level as a datum line, 

 are stated near the end of this article, where the physical changes of 

 the sea are noticed. But though the surface of the sea is a regular 

 curve, minute investigation has shown that there are some irregu- 

 larities, and that some parts of the sea are more elevated than others. 

 This is particularly the case with closed seas, which are generally more 

 elevated than the ocean. The level of the closed seas is higher than 

 that of the ocean when the mass of water brought to them by the 

 rivers which discharge into their basin is greater than that which is 

 lost by evaporation, and the straits by which they are united to the 

 ocean are not wide enough to carry off the surplus waters quickly. 

 On the other hand, when the evaporation is greater than the supply of 

 water from rivers, the level of the closed sea sinks below that of the 

 ocean, and it must be supplied with water from the latter by the straits 

 which unite them. 



The Baltic, though of no great extent, and though united to the 

 open sea by three straits, one of which is of considerable width, 

 receives so great a supply of river-water, that its level is higher than 

 that of the North Sea. Very exact measurement has shown that this 

 difference amount* to more than a foot between the level of the North 

 Sea near the mouth of the river Eider and that of the Baltic near the 

 town of Kiel. It is true that when the level of the North Sea has 

 been raised by a continuance of western and north-western winds, a 

 current sets from the Cattcgat into the Baltic, but in calm weather it 

 is always found that the current seta northward through the three 

 straits. The difference of level between the Black Sea and the Mediter- 

 ranean is much greater. The large rivers which fall into the Black Sea 

 bring down an immense volume of water; and accordingly a very 

 strong southern current is constantly found to be setting southward 

 through the Strait of Constantinople into the Sea of Marmora. It 

 generally runs with a velocity of about three miles an hour, which 

 however at one place, called the " Devil's Current," is much greater, 

 and at times between five and six miles per hour. The velocity of 

 tli is current must vary with the seasons ; for it is stated that the level 

 of the Black Sea in winter is between two and three fathoms higher 

 than in summer. The Sea of Marmora, which thus receives the 

 mil-plus of the waters of the Black Sea, must also be more elevated 

 than the Mediterranean ; for the current which sets through the Strait 

 of the Dardanelles is likewise constant and rather quick, though not so 

 quick as that in the Strait of Constantinople. 



The Mediterranean, on the other hand, receives a very scanty supply 

 of water by rivers ; for with the exception of the Nile, no large stream 

 falls into its basin, which is of much greater extent than that of other 

 closed seas, and therefore it must lose a great volume of water by 

 evaporation. Halley showed that the Mediterranean, whose tempera- 

 ture is from 4* to 5 Fahr. higher than that of the Atlantic under 

 the same latitude, must lose by evaporation nearly three times as much 

 water as is brought into it by the rivers. The deficiency is supplied in 

 two ways : by the current of the Dardanelles, which brings to it the 

 surplus waters of the Black Sea and of the Sea of Marmora ; and by 

 that which sets through the Strait of Gibraltar from the Atlantic 

 Ocean. The Atlantic current runs somewhat more than one mile and 

 a half per hour. It has been supposed that, though this current con- 

 stantly sets into the Mediterranean, an under-current runs in an oppo- 

 site direction, carrying back a portion at least of the water to the 

 Atlantic ; but the attempts which have been made to establish this 

 fact have failed, and it is probable that this supposed under-current 

 does not exist. Notwithstanding the large supply of water which the 

 Mediterranean receives at its two extremities, the level is below that 

 of the Atlantic. Corabomf found that the difference of level on the 

 Mediterranean near Perpignan, and on the Bay of Biscay near Bayonne, 

 amounted to nearly six feet ; and Delambre and Mechain found it to 

 be nearly three feet between the North Sea at Dunkerque and the 

 Mediterranean near Perpignan. 



In these instances the difference of level is satisfactorily explained;; 

 but the explanation is not so easy with respect to the great differ- 

 ence between the level of the Mediterranean and that of the Red 

 Sea. These two seas are separated by the Isthmus of Suez, which 

 extends about 70 miles from north to south. When the French 

 occupied Egypt, they executed an extensive levelling across this 



isthmus; and the result was, that the Red Sea is above 32 feet 

 higher than the Mediterranean. No river of importance, not even a 

 perennial stream, falls into the Red Sea, which must also lose a con- 

 siderable volume of water by evaporation. This loss of water is 

 probably supplied by the current which sets into the Red Sea from 

 the Indian Ocean ; and some persons are of opinion that the difference 

 of level between the Red Sea and the Mediterranean might be pro- 

 duced by this current. But it seems improbable that such an effect 

 can be produced by this cause. According to Horsburgh and Wellsted 

 (' Journ. of Lond. Geogr. Soc.,' voL vi.), a current sets from the 

 Indian Sea into the Red Sea between October and May, and it often 

 runs with great rapidity. But between May and October the northern 

 winds prevail through the whole extent of the Red Sea ; aud these 

 winds, which frequently blow a gale, cause a continual current to set 

 through the straits into the Gulf of Aden. Under such circumstance's, 

 it is evident that the sea must fall to its natural level, especially as 

 this state of things continues for more than three months. "Wellsted 

 observes that in this season, from May to October, the reefs in the 

 northern part of the Red Sea have about 2 feet less water on them 

 than in the remaining months of the year. This therefore appears to 

 be the whole extent of the difference produced on the level of the 

 Red Sea by the current, which enters it through the Strait of Bab-el- 

 Mandeb from October to May ; but the French, as already observed, 

 found the difference between the levels of the Red and Mediterranean 

 seas to be not less than 32 feet. It would therefore appear that the 

 Indian Ocean itself must be about 30 feet higher than the Mediter- 

 ranean, and probably also higher than the Atlantic near the Strait 

 of Gibraltar (see below) and that the difference of level in the different 

 parts of the ocean is much greater than is commonly supposed. 

 This is also proved by the difference of level between the Atlantic 

 and the Pacific on both sides of the Isthmus of Panama. Accord- 

 ing to Lloyd (' Phil. Trans.,' 1830), the mean rise and fall of the 

 Pacific two days after full moon is 21*22 feet, and in the Caribbean 

 Sea 1*16 feet. The water at high-water mark in the Atlantic is 13'55 

 feet lower than in the Pacific. The mean between the high and low 

 water in the Pacific is 10'61 feet, and in the Atlantic 0'58. It would 

 therefore appear that the level of the Pacific is 3'52 feet higher than 

 that of the Caribbean Sea, as at low-water, two days after full-moon, 

 the Pacific sinks 6'51 feet below the level of the Atlantic ; but it rises 

 at high-water 13'55 feet above it. 



In this comparison of the level of the two oceans, the Caribbean Sea 

 is placed in opposition to the Pacific : but in reasoning from existing 

 data, and our present knowledge, we must admit that the level of the 

 Caribbean Sea is much higher than that of the Atlantic near the 

 old Continent. The north-eastern and eastern trade-winds force a 

 great volume of water from the North Atlantic into the Caribbean 

 Sea, and this is increased by another large volume of water which 

 is brought to that sea by the Guiana current, and which enters it by 

 the straits between the islands of Martinique and Trinidad. 

 [ATLANTIC OCEAN, in GEOO. Div.] Such volumes of water, being 

 arrested by the long isthmus which separates the Caribbean Sea and 

 the Gulf of Mexico from the Pacific, must produce a considerable 

 accumulation of water along the western shores of those seas, and 

 raise them above the common level of the Atlantic, and this fact is 

 confirmed by the rapid current called the Gulf stream. [ATLANTIC 

 . in GEOO. Div.] Opinion varies considerably aw to the differ- 

 ence of level between the Gulf of Mexico and the Atlantic. Poussin 

 found tlie level of the Gulf at the mouth of the river Suvvanee 

 3-75 feet higher than that of the Atlantic at the mouth of St. John's 

 River in Florida : but when Darby, in his ' View of the United 

 States,' estimates the difference between the Gulf near the island of 

 Cuba and the entrance of Chesapeake Bay as at least 83 feet, we 

 must suppose that he has formed his opinion on erroneous data. 

 Also, we cannot accede to the opinion of Humboldt, who, in compar- 

 ing some barometrical observations made at Cumana, Cartagena, aud 

 Vera Cruz, with others made at Acapulco and Callao, came to the con- 

 clusion that in these parts the Pacific was about 9'5 feet lower than 

 the Caribbean Sea and the Gulf of Mexico. Lloyd found the reverse 

 to be the case at the mouth of the Chagres and at Panama, as we have 

 mentioned above. 



Mathematical investigation, founded on modern exact physical 

 research, has however led to the discovery at once of a cause of 

 difference of level in the sea, very different from any of the causes 

 considered above, and of an example of that difference of the most 

 interesting and instructive character. In the ' Philosophical Transac- 

 tions ' for 1859, p. 779-796, is a paper by Archdeacon Pratt, ' On the 

 Influence of the Ocean on the Plumb-line in India,' which is a sequel 

 to two former communications on the effect of Mountain Attraction on . 

 the Plumb-line in India. In this paper the author first describes the 

 remarkable nature of the geographical position of Hindustan, con- 

 sisting in this : that the liighest mountain-ground in the world lies 

 to the north of it, and an unbroken expanse of ocean extends from its 

 shores down to the neighbourhood of the South Pole. Now as water 

 is lighter than land, that is, the quantity of matter contained in a given 

 bulk of water is less than that contained in an equal bulk of land, the 

 attraction or attractive power of the ocean is smaller than that of the 

 land. A given bulk of laud has a greater attractive power than an 

 equal bulk of sea. Therefore, the sea on the shores of Hindustan is 



