8 rUTSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



24. Southern seas the boiler, and northern lands the condenser. — 

 These facts have suggested the comparison in which the southern 

 hemisphere has been likened to the boiler and the northern to 

 the condenser of the steam-engine. This vast amount of steam 

 or vapour rising up in the extra-tropical regions of the south, 

 expels the air thence, causing the barometer to show a much less 

 weight of atmosphere on the polar side of 40^ S., than we find in 

 corresponding latitudes north. 



25. Offices of the atmosphere. — The offices of the atmosphere ai-e 

 many, marvellous, and various. Though many of them are pa.st 

 finding out, yet, beautiful to contemplate, they afford most 

 instructive and profitable themes for meditation. 



26. Dr. Buist. — ^^ hen this system of research touching the 

 physics of the sea first began — when friends were timid and co- 

 labourers few, the excellent Dr. Buist stood up as its friend and 

 champion in India ; and by the services he thus rendered, en- 

 titled himself to the gratitude of all who, with me, take delight 

 in the results which have been obtained. The field which it 

 was proposed to occupy — the firstlings of which were gathered in 

 this little book — was described by him in glowing terms, and 

 Avith that enthusiasm which never fails to inspire zeal. They are 

 apropos, and it is a pleasure to repeat the substance of them. 



27. The sea and the atmosphere contrasted. — " The weight of the 

 atmosphere is equal to that of a solid globe of lead sixty miles in 

 diameter. Its principal elements are ox3'gen and nitrogen gases, 

 with a vast quantity of water suspended in them in the shape of 

 vapour, and commingled with these a quantity of carbon in the 

 shape of fixed air, equal to restore from its mass many fold, 

 the coal that now exists in the world. In common Avith all sub- 

 stances, the ocean and the air are increased in bulk, and, con- 

 sequentl}^ diminished in weight, by heat ; like all fluids, they 

 are mobile, tending to extend themselves equally in all directions, 

 and to fill up depressions Avherever vacant space will admit them ; 

 hence in these respects the resemblance betwixt their movements. 

 Water is not compressible or elastic, and it may be solidified into 

 ice, or vaporized into steam ; the air is elastic ; it may be con- 

 densed to any extent by pressure, or expanded to an indefinite 

 degree of tenuity by pressure being removed from it ; it is not 

 liable to undergo any change in its constitution beyond these, by 

 any of the ordinary influences by which it is affected. 



28. Influence of the sun. — " These facts are few and simj)lo 



