90 Notices of Memoirs — 



A further confirmation of these views is obtained from a comparison 

 of specific gravities. The density (due to salinity) of surface-water 

 increases from the poles to the tropics, while that of bottom-water 

 in the tropics is nearly the same as in the polar area. Why then 

 does the bottom-water of the tropics, being of lower salinity, under- 

 lie the more saline strata ? Because the density it lacks from its 

 lower salinity is more than compensated by the lowness of its tem- 

 perature. Passing, however, from either tropic towards the Equator, 

 the salinity of surface-water is found to diminish, until its specific 

 gravity is reduced from 1027-3 to 10264 or 1026-3, which is that of 

 the polar under-flow. Lenz adduced the low salinity of the surface- 

 water under the Equator as evidence of the rise of polar water from 

 the bottom, and showed that there is a band of water at the Equator 

 colder than any to the north or south of it. 



The Oceanic Circulation thus produced brings every drop of water 

 in turn to the surface, enabling it to part with carbonic acid and to 

 absorb oxygen ; this, then, is its importance to Animal Life. From 

 the analysis of gases dissolved in the water of the oceanic area, it 

 was found that, for 15 per cent, of carbonic acid, there was usually 

 from 16 to 20 per cent, of oxygen — this being the result of a series 

 of observations taken off Ireland and Scotland at various depths 

 down to 2000 fathoms. This amount of oxygen is sufficient to support 

 a large quantity of Animal Life, in spite of the, to air-breathers, 

 fatal proportion of carbonic acid — if indeed the carbonic acid be 

 not in a liquefied, and thus perhaps more innocuous form. 



In the Mediterranean totally different conditions prevail. It was 

 expected that a Tertiary fauna would be found at great depths, 

 analogous to the Cretaceous-like fauna of the ocean outside. Instead 

 of that, only a viscid mud, almost devoid of life, was brought up. 

 The western basin has a depth of 1600 fathoms, the eastern basin 

 one of 2000 fathoms ; the bottom temperature is nearly uniform at 

 about 55° F., a great difference in thermal condition from the 

 Atlantic. The reason is that the Mediterranean is cut off entirely 

 from the polar under-flow, which, off Lisbon, produces a temperature 

 of 40° E. at a depth of 700 fathoms, and 36|° at 1500 fathoms. In the 

 Mediterranean, on the other hand, we have a surface temperature from 

 60° to 70° F., which, in the first 100 fathoms, falls to 54° or 55° F., 

 below which to the bottom, no matter at what depth, there is no change 

 at all, but a slight variation according to latitude, due in part to the 

 mean winter temperature of the locality. The whole of the lower 

 portion, therefore, below the influence of the Gibraltar current, is a 

 mere stagnant pool ; and this is the explanation of the absence of 

 Animal Life except in the shallows. The impalpable mud, which is 

 slowly settling to the bottom, may also not be without its effect. 

 This is the result of the attrition of soft Tertiary shores, and of the 

 clay brought down by the Ehone into the western basin, and by the 

 Nile into the eastern, the finer particles pervading the entire sea. 

 Corals and Bivalves suffer from it especially, The per-centage of 

 carbonic acid was found to be as high as 60, whilst that of oxygen 

 was only 5 ; this is believed to be due to the organic matter, brought 



