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 are many, marvellous, and various. Though many of them are past finding out, yet, beautiful to contemplate, they afford most instructive and profitable themes for meditation.
26. Dr. Buist.—When 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, entitled 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 with 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 oxygen 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 with all substances, the ocean and the air are increased in bulk, and, consequently diminished in weight, by heat; like all fluids, they are mobile, tending to extend themselves equally in all directions, and to fill up depressions wherever 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 condensed 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 simple
