What is the actual mean level of the sea f By the researches of Captain Denham at Liverpool, by the careful measurements taken by Mr. Bunt in the Bristol Channel, under the direction of Professor Whewell, and by some experiments of Sir H. de la Beebe on the coasts of Pembrokeshire, it appears that the mean of high and low water is nearly at the same point, whether spring or ucap tides be measured. This mean or half-tide level, which is often nearly coincident with the middle point of time between high and low water, can be ascertained by a few observations at any age of the moon in quiet weather, and should always be employed as the datum or zero line in recording elevations of mountains, heights of canals, summits, railway stations, fie. Above all it should he taken for the term of comparison whenever it is wished to determine the relative elevation of different parts of the open sea, though there may be cases when landlocked waters and tide rivers may require the use of another line of reference. The horizontal line which bounds the land and sea, the outline of the coast, is variable ; it is annually displaced by the wasting of some tracts and the augmentation of others. Since the Roman sway in Britain, a large portion of the rich marsh-lands of Norfolk, Cambridge, Huntingdon, &e., have been recovered from the retiring sea ; but in the same period the east coast of Yorkshire has lost many square miles of territory, and is still losing at a fearful rate. (Sir C. Lyell has collected abundant examples in his Principles of Geology.') This unequal action depends partly on the slope with which the land and sea meet ; partly on the nature of the materials comprising the coast, partly on the set of the tides and currents.
The modern ocean is the theatre of many operations similar in kind to those by which the materials of dry land were formerly accumu lated beneath ancient seas. The bed of the Adriatic was found by Donati to resemble the surfaces of sub-Apennine tertiary strata ; the shallow soundings of the English Channel and German Ocean show, in the distribution of shells and fish-teeth, analogies with several of the secondary rocks ; while in the coral reefs of warm latitudes, on sandy or pebbly shores generally, and at the mouths of great rivers, we see the production of limestone, sandstone, and clay deposits, very similar to those which abound in the stratified masses of land. Whoever will compare with attention the ripple or current mark on the sea-shoro with the corresponding undulations on slates and sandstones of every geological age, will be convinced of the identity of the causes of these impressions ; and when he beholds such surfaces in rocks covered by other deposits thousands of feet thick, will not hesitate to admit in such eases that great depressions happened along the margin of the ancient sea during the formation of these strata, followed by still greater elevations at a later period.
The distribution of life in the modern ocean is one of the circum stances moat important to know. Until very recently it has been sup posed that below some moderate depth (moderate at least as compared with the thicknesses of the strata) life ceases in the ocean from deficient light and air, and augmented pressure.
- Within these few years, however, reason has been seen to modify thee* oonclusions ; and in the course of the past year (18C0) the researches of Dr. G. C. Wallich, on the bed of the North Atlantic, have ' shown that animal life, in various forms of foraminifers and radiate, if net of higher group's, exists at vast depths. It ought, indeed, to hare been seen long ago, that the uniform distribution of pressure effected by the water, would necessarily preclude the destruction which it was reasonable to infer that the partial accumulation of pressure would have caused.
The proportion which exists between the sea and land has contri buted to maintain the productive powers of the earth. If that pro portion were materially changed, its productive powers would be changed also. The sea, by means of the vapours continually rising from its surface, supplies the atmosphere with sufficient moisture for the support of organics life. Countries which do not partake of the enefita derived fn m this source, and which are not refreshed by rain or dew, are uninhabitable and destitute of all kinds of vegetation.
Those parts of the earth which are farthest from the sea are much leas fertile and populous than those which, owing to their greater vicinity to it, receive a larger supply of moisture from this great source. The sea contributes aho considerably to the advancement of civilisation.
At the first view it seems to constitute an insuperable obstacle to tho communication between nations who inhabit countries widely apart from one another; but tho ingenuity of men has converted the ocean into the most frequented high road on the globe. The easy commu nication which is thus established between nations at great, distances from one another, has perhaps more than any other circumstance con tributed to improve the condition of the human race. It is at least certain, that all those nations which have acquired any considerable degree of civilisation inhabit countries either contiguous to the sea or at no groat distance from it.
On the subjects of this article, four works may be particularised, which, of course among many others, may be consulted with advan tage : Sir John F. W. I Iersehel's • Physical Geography ;' Admiral Sioytlie ' Mediterranean ;' Sir John Itichardsou's ' Polar Regions ;' and • The Physical Geography of the Sea, and its Meteorology,' by Captain Maury, the American hydrogmpher. The last is an immense repository of facts, relating chiefly, however, to the Atlantic and its marine depen dencies ; and many views entertained by tho author are not wholly accepted by British hydrographers and meteorologists.