413 



SEA. 



SEA. 



411 



are generally destitute of harbours, or at least they occur only 

 at great distances, and much expense is required to maintain them in 

 an efficient state : the harbours on rocky coasts are not subject to this 

 inconvenience. 



It is a very remarkable phenomenon, which has not yet been satis- 

 factorily explained, that the temperature of the sea decreases as the 

 land is approached, and it also decreases on shoals and banks ; and as 



crease may be detected by the thermometer at a considerable 

 distance from land, this instrument is now used for the purpose of 

 ascertaining the approach to land or the presence of shoals. It must, 



r, be observed, that though this decrease generally takes place, 

 it in not universal. 



As late as the end of the last century it was a generally received 

 opinion that the whole mass of sea- water, from the surface to the 



i, had the same temperature in the same latitude. But nume- 

 rous observations, which have been more recently made, have shown 

 the inaccuracy of this assumption. It has been found that the law 

 which is constant for the earth must be inverted for the sea. The 

 farther we descend into the interior of the earth, the higher is the tem- 

 perature ; but the deeper we dip into the sea, the lower is the tempe- 

 rature of the water. But this does not take place in the same ratio in the 

 whole sea. Captain Ross found the temperature of the sea in Baffin's 

 Bay, 3900 feet below the surface, + 25'52 , while the surface itself was 

 + 33-80*. Beechey, in 47 18' N. lat., at a depth of 5124 feet, found 

 the water + 89-56, and at the surface it was 46'22. Sabine found 

 that the water in the Caribbean Sea, at a depth of 1000 fathoms, had 

 a temperature of 45'Sii", whilst at the surface it was 83*30. According 

 to an observation of Franklin, the water at a depth of 650 fathoms, in 

 57 44', wa 40'5; at 450 it was 41 ; and at the surface, 45. There 

 are a few instances in which a new increase of the temperature has 

 been observed at a very great depth. In the following observations 

 nude by Beechey, in the Pacific, the temperature became stationary at 

 great depth : 



2S 28' If. lat. 

 Temp, at the surface 

 At a depth of 300 ft. 



;.th of 900 ft. 

 At a depth of 1 260 ft. 

 At a depth of 1 860 ft. 



6J- 1 



SO' 



4T'-i 



47'-S 



48" 48' JV. lat. 

 Temp, at the surface 

 At a depth of GOO ft. 

 At a depth of 1200 ft. 



54' 

 45 

 4T-5 



At a depth of 1962 ft. 40'-5 

 At a depth of 2652 ft. 40 1 -5 



As a proof of the increase of the temperature of the sea at a great 

 depth, we copy the following observations, of which the first two were 

 made by Beechey, and the last by Prescott ; the first in the Pacific, 

 and the last two in the Atlantic Ocean : 



14'S2'.V. lat. 



Temp, of the air 91 

 Of the iea at it ratface 88 

 At a depth of 600 It. 



At a depth of 1100 ft. S3' 



At a depth of 1800 ft. 4SV. 



At a depth of 2400 ft. 41) -, 





45' S8' S. lat. 



Temp, of the air 37' 



Of the sea at it* turface 43'-S 

 At a depth of 600 ft. 



At a depth of 1380 ft. 42'-5 



At a depth of 1980 ft. i 



At a depth of 1580 ft. 41-:. 



.V. lul. 



Temp, of the air 83' 



Of the wa at iu tarfaee 83 

 At a depth of 180 ft. 71' 

 At a depth of 160 ft. 81' 



At a depth of 540 ft. 

 At a depth of 660 ft. 

 At a depth of 720 ft. 



47' 



48' 

 41 



At a depth of 1320 ft. 60' 



The renewed increase at a great depth is a very difficult problem to 

 solve : it is possible that it may be owing to submarine currents of 

 different degree* of temperature, as some have supposed. Sii Jann * 

 < ascertained, by seven different experiments, that between the 

 parallel* of 65 and 58 30' S., there is a belt encircling the earth, 

 where the mean temperature of the sea, that of the greatest density of 

 Ha wter, or -t- 89'5, prevails throughout its entire depth, forming a 

 ill ill border between two great thermic basins of the oceun. 



But there are facto on record which clearly show that in certain 

 |u>rU of the ocean there must exist some agency by which the water 

 acquire* a higher degree of temperature than might have been expected 

 from ordinary caiises. Homer, in Knuenstern's ' Travels,' observes 

 that, in some place* in the Gulf-stream, the hand-lead, when it had 

 descended to a depth of between 480 and 600 feet, was heated to such 

 a degree that it wan impossible to take it into one's hand. A still more 

 remarkable anomaly is presented by the temperature of the sea between 

 Greenland and Spitsbergen. In nearly every trial, gcoresby found that 

 this wa, at a depth of from 100 to 200 fathoms, was from 6* to 7 

 warmer than at the surface ; and Franklin state* that when he accom- 

 panied Captain Buchan in his expedition to the North Pole, the water 

 brought from any great depth was invariably found to be warmer than 

 that of the surface. Home persona are of opinion that the melting of 

 the great maawa of ice, by which that sea is surrounded and partly 

 covered even in summer, may have had the effect of cooling the surface. 

 But this is contrary to the well-established law that the colder water, 

 being the denser, sinks to the bottom, and the warmer rises to the 

 surface ; and further, it may be asked why Ross and Parry, in navi- 

 gating ' 'I'iffln'a Bay, and Hudson's Bay, where the masses 

 of ice are neither less numerous nor less extensive, always fuund the 

 contrary to take place. 



The important subjects of the distribution of temperature in the 

 atmosphere over the sea, and of the mutual relations of the temperature 

 of the two elements, will be considered iu the article TEMPERATURE, 

 TERRESTRIAL, DISTRIBUTION OF. 



Masses of ice surround the two poles. The cause of this has been 

 thus generalised by Sir J. F. W. Herschel :" Beyond the 56th 

 parallels of latitude, the temperature of the water is lower at the 

 surface, and rises as the thermometer descends, till the level of 39 [or 

 the temperature of greatest density] is reached. The sea, then, even 

 in deep water, becomes frozen at the surface in the winter months, or 

 rather through all that portion of the year which intervenes between 

 the beginning of September and the latter end of June ; July and 



August being, in high latitudes, the only open months There 



can be very little doubt that, in the winter time, the surface of the 

 ocean at both poles is entirely frozen ; but at the North Pole it is 

 probable, from many indications, that open water exists over a very 

 large area of the central polar basin during a considerable portion of 

 the warmer months." borne of these indications are then stated, but 

 a fuller new of them will be found in Sir John Richardson's ' Polar 

 Regions,' pp. 222-231. 



It was formerly conjectured that the mass of ice inclosing the 

 North Pole extended to the vicinity of 81 N. lat., because all navi- 

 gators who had attained that latitude agreed in stating that the ice 

 there rose to a great height, and stood firm like a wall. This general 

 opinion gave rise to the attempt of Captain Parry to reach the pole by 

 travelling on this ice, which was supposed to be immovable. But 

 Parry was soon aware that he was travelling on ice which was in con- 

 tinuous motion, being carried by a current towards the south and 

 south-west, and this circumstance occasioned the failure of the under- 

 taking. When he had advanced, according to his calculation, several 

 miles to the northward in twenty-four hours, he found, on observing 

 the altitude of the sun, that the motion of the ice had carried him as 

 far southward, aud that for several days he had advanced very little 

 nearer to his object. He was obliged to abandon the attempt, after 

 having reached 82 404' N. lat. Thus we have learned that the 

 exterior parts of the great mass of ice supposed to inclose the poles 

 consists of moving masses, which lie close together, and are only occa- 

 sionally divided from one another by narrow straits. The pieces of ioe 

 which detach themselves from this great accumulation and enter the 

 open sea are called heaey drift-ice. The larger pieces of ice of this 

 description are a mile in length and breadth, and upwards of 30 feet in 

 thickness ; but others are of less dimensions. The farther they advance 

 southward, the more their dimensions are reduced by the action of the 

 sun and of the water. But there are two other descriptions of ice- 

 masses in the sea, which appear to have a different origin the ice-fields 

 and the icebergs. The terra ice-Jield ia applied to sheets of ice so 

 extensive that their limits cannot be discerned from the mast-head. 

 They often occur of the diameter of 20 or 30 miles, and, when they 

 are very closely united, they sometimes extend to the length of 50 or 

 100 miles. Their average thickness may be from 10 to 15 feet, and 

 their surface is mostly level, except where hummocks or low ice-hills 

 occur, and then the thickness ia often 40 and even 50 feet. These 

 hummocks are produced by two fields coming into contact, when their 

 broken edges are raised by the violent concussion, and thrown upon 

 the fields themselves. These hummocks, therefore, are usually situated 

 near the edges of the field. In some fields the hummocks form ridges 

 or chains ; in others they consist of isolated peaks. The smaller fields, 

 or those whose extent can be seen from the mast-head, are called Jloee. 

 The Mirl'ace of these masses of iee, before July, is always covered with 

 a bed of MIUW from a foot to a. fathom iu depth ; this snow dissolves in 

 i , and forms extensive pools aud lakes of fresh water. 

 The great extent and the level surface of the fields show that they 

 cannot be portions of the ice over which Parry travelled. It is there- 

 fore supposed that they are generated in the sea which lies between 

 (irionlanil and .Snitzbergen, and which, though navigable during the 

 summer, is r<>vriv.l with a continuous sheet of ice in the colder season. 

 The fields appear to bo the parts of this great sheet, formed by its 

 breaking up at the approach of summer. When, on the further 

 advance of the season, the field-ice and floes of the general surface 

 break, the ice becomes heaped together in sheets confusedly piled on 

 each other, into what is called i an ice-pack. The pellicle of 



ice formed over great areas of the polar seas, as the immediate efl'ect of 

 a general depression of temperature below th^ freezing-point of sea- 

 water, is called pancake-ice. It is, in fact, the beginning of the pro- 

 duction of icn in the ocean ; and the possibility of its existence, as 

 well as that of all continuous masses of ice, depends on the property of 

 revelation, to which Faraday has recently called attention [!CK ; 

 \\ ATI it], aud which Tyndall has applied with so much success to the 

 elucidation of the phenomena of glaciers, the parents of iceljer;/*, to 

 which we must now proceed. These are immense masses of ice rising 

 to a great height above the level of the sea ; some of them attain a 

 height of 100 feet above the surface of the sea, and a few have been 

 found which seem to be more than twice that height. Their base near 

 the sea-level is not extensive, the larger masses generally being not 

 more than 4000 feet in circumference, though Middleton states that he 

 Haw one which was from three to four miles in circuit. The most 

 i-omiiii>n form of the iceberg is for one side to rise perpendicularly to 

 the very summit, the opposite side being very low, while the inter- 



