Voijages of Discovery and Survey. 321 



ing, 130 feet above the water, in about latitude 58° 36' S. As the 

 icebergs passed into regions where the decay, in the atmosphere, of 

 the higher portions of them became more considerable than of those 

 beneath, they would cease to upset, and would carry their loads of mud 

 and stones uppermost or below, according as they may have been 

 upset more than once by remaining a sufficient time within the need- 

 ful conditions. 



Whenever opportunities occurred. Sir James Ross was indefati- 

 gable in trying for soundings and the temperature of the sea at dif- 

 ferent depths. The results are highly valuable. He was enabled 

 to ascertain that a belt of sea of uniform temperature, from its sur- 

 face to the greatest depths, extends round the southern regions in a 

 mean latitude of about 56° 26'. Though this may be the mean, it 

 was found to vary in position from 58° 36' in longitude 101° 40' W. 

 to 54° 41' in longitude 55° 12' W., being a difference of 3° 55' of 

 latitude. Such variations are to be expected from local causes, and 

 even in the same locality from modifications due to great changes of 

 seasons. Sir James Ross points out that this belt forms a barrier 

 between two great thermic basins, the temperature of 39°'5 (that of 

 the most dense sea-water, according to the observations made durino- 

 this voyage), descending on the north of it to the depth of 3600 feet 

 in latitude 45° S., and in the tropical and equatorial regions to that 

 of 7200 feet, the surface temperature being 78°, while in latitude 

 70° S. the line of uniform temperature descends to 4500 feet, the 

 surface temperature being 30°. 



When we compare the distances with the depth of uniform tem- 

 perature here noticed, and assume, for the sake of easy illustration, 

 a level plain from the equator to latitude 70° S., we find that from 

 the surface belt of 39"' 5, the inclination of this line of temperature 

 would be to the equator on the one side about 1 in 1723, and to 

 latitude 70° about 1 in 1136 on the other. Thus the slopes would 

 be most gradual, and the depressions on the north and south so slight, 

 compared with the distances, that a tulerably long section would shew 

 these two thermic basins as slight depressions, and in a section less 

 long, as scarcely distinguishable from a thick line. 



With such a section before us, we experience little surprise that 

 the tendency to occupy the same relative level, from the greatest 

 density, should be greatly modified by the atmospheric influences on 

 the surface of the ocean. 



These observations respecting a belt of uniform temperature in the 

 ocean in the southern heiuisphere, would lead us to anticipate that, 

 similar causes l>eing in action in the northern hemisphere, similar 

 results would be found there, though no doubt modified by the pre- 

 valence of land in the north as compared with the south. The tem- 

 perature of 39°"5, obtained by Sir James Ross for the apparent 

 greatest density of water in the ocean, is not that which experiments 

 ill the cabinet would have led us to expect. Dr Marcet found that 



