HA RD WICKE 'S S CIENCE ■ G O SSI P. 



Si 



and round at the top, instead of terminating in a sharp 

 point, as when the animal is full grown. 



Having now shown (as I think conclusively) that 

 the harvestman is not a spider, will some one tell me 

 what it is ? 



Xorwich. J. H . Gary. 



THE MIGHTY DEEP. 



IN a general way persons have a better appre- 

 ciation of the vastness of the land than of that 

 of the sea ; mainly because the former is more fre- 

 quently forced on their attention. The area of the 

 ocean is nearly thrice that of the land ; the one 

 being estimated at about 52 million square miles, 

 and the other at 145 million square miles. On the 

 land, as well as in the ocean, there are vast tracts on 

 which life is absent or scanty ; but whereas, on the 

 land the inhabitable portion is to a great extent 

 superficial, in the ocean life occurs more or less I 

 abundantly at all depths. The ocean forms one 

 continuous mass of water broken up into irregularly- 

 shaped portions by the land. When portions of 

 the sea are inclosed by the land, the water under- 

 goes so marked a change in character that such 

 inclosed portions (forming lakes and lagoons) can 

 no longer be considered as forming part of the 

 ocean. The sjreat bulk of the sea is concentrated in 

 the South hemisphere, and the pole of the sea (that 

 is, the centre of the hemisphere in which it is most 

 extensive) is in 52 S. 6° E. In this hemisphere 

 the land is to sea as 1 to 8, while in the opposite 

 hemisphere there is nearly as much land as sea. 



The mean depth of the sea has been variously 

 estimated, but as yet there are scarcely sufficient data 

 for accurate estimates. Buffon suggested it might 

 be 200 fathoms or 1,200 feet ; Lacaille, 163 to 273 

 fathoms; Laplace, 656 fathoms ; Lyell, 2,600 

 fathoms ; and Herschel, 3,520 fathoms. The most 

 probable mean is about 2,600 fathoms. Assuming 

 this as correct, the average volume per square mile 

 would be 418, 176 million cubic feet, and the total 

 volume 60,635,520,000,000 million cubic feet. The 

 mean height of the land is 1, 000 feet above the sea-level, 

 which would give a volume of 1,449,676,800,000 

 million cubic feet for the supramarine portion. 

 Hence the ocean's bulk is 41 times greater than that 

 of the land above its upper surface. 



One of the causes now in operation which tend to 

 make the ocean encroach upon the land is the intro- 

 duction of detrital matter into the sea. Every grain 

 displaces its own bulk of water, and so far causes it 

 to encroach upon the land. Denudation is always 

 going on at a probable mean rate of one foot in 

 3,600 years, at which rate all the land would be 

 removed in about ten million years. The sea, in 

 that case, would gain on the land at the mean rate of 

 about five square miles per year. If all the land 



were transferred to the sea, the mean depth of the 

 latter would be reduced to 1,500 or 1,600 fathoms. 

 Other causes, however, may influence the extent of 

 the ocean. One is the subsidence of land below the 

 sea-level and elevation or subsidence of the sea 

 bottom. The subsidence of the whole of the land 

 would involve a depression of about 30,000 feet ; 

 and the elevation of the whole of the sea bottom 

 would require an uplift of about 35,000 feet. In all 

 probability elevation and subsidence proceed simul- 

 taneously in different parts of the earth, and may or 

 may not counterbalance each other. The bearing of 

 this conjecture is that extensive subsidence or eleva- 

 tion of the sea bottom is calculated to be more in- 

 fluential than the introduction of sediment in causing 

 the sea to advance upon or withdraw back from the 

 land. The introduction of a mass of matter equal in 

 bulk to all the supramarine land would be sufficient 

 to cause the ocean to overflow the land up to about 

 the level of 6,000 feet above the present level ; while 

 alterations of level less than those of which the 

 geologist is cognizant, as local phenomena at least, 

 would, if extended over large, areas account for the 

 displacement of entire continents and oceanic basins. 

 It has been assumed that the bulk of the ocean has 

 been approximately unaltered; but surmises might be 

 made upon the variations in the amount of water 

 which, in its aeriform and liquid states, is temporarily 

 withdrawn from the sea. A rough calculation indi- 

 cates that the average amount of vapour constantly 

 in the air corresponds to about 13,412,704 million 

 cubic feet of water, and that the amount of fluid re- 

 quired to keep all lakes, rivers, &c, supplied for six 

 months is about 2,364,072,004 million cubic feet, or 

 about sufficient to form 5,894 square miles of sea of 

 average depth. The entire absence of all flowing 

 fresh waters, or a doubling of their present volume 

 (extreme conditions which are not likely to have hap- 

 pened), would have no appreciable influence geolo- 

 gically in altering the relative areas of land and sea. 

 The accumulation of snow and ice would perhaps be 

 more influential. For, supposing it possible that at 

 one time there were no ice, and that at another some 

 10 million square miles were covered therewith to a 

 depth of 500 feet, this would give a volume of 

 139,392,002,000 million cubic feet, which cor- 

 responds to 309,071 square miles of sea of average 

 depth. If we assume that the sea has retained its 

 mean depth unaltered, the utmost probable irregu- 

 larity in the amount of "rainfall and in the accumula- 

 tion of ice upon the land would not cause its area to 

 vary by so much as one million square miles. Hence, 

 from a geological point of view, the possible influence 

 of such irregularities may be disregarded. There is 

 yet another influence to be noticed. It is exceed- 

 ingly probable — nay, we may say certain — that the sea 

 bed consists of material which is in places permeable, 

 and in others impermeable, to sea water. At what 

 rate the water percolates into the rocks, and what 



