Oct. 15, 1885 | 
NATURE 
583 
cut off from direct communication with the ocean. These lakes 
differ as much from the waters of the ocean as do the oceanic 
islands from the land of the continents. 
The surface of the earth may then be divided into three great 
regions—the abysmal area, occupying, so to speak, the bottom 
of the basins, covering one-half of the earth’s surface ; a border 
region occupying, so to speak, the sides of the basins, covering 
three-sixteenths of the earth’s surface ; and lastly, the continents 
which cover five-sixteenths of the earth’s surface. The average 
height of the elevated plateaux of the continents above the 
submerged plains forming the abyssmal regions is fully three miles. 
When we pass to a consideration of the water of the ocean, 
which fills these great hollows of the earth, it is essential to take 
account of the superincumbent atmospheric ocean, which every- 
where rests on its surface, for the composition of the ocean water, 
the currents, the distribution of salinity, density, temperature, 
and even that of deep-sea deposits, are largely determined by 
the movements of the atmosphere. 
One of the most important parts played by the ocean in the 
economy of the globe is to bring about a more equable distribu- 
tion of temperature by the winds which blow from it over the 
land and by means of the oceanic currents that are originated 
and maintained by the winds. 
From the smallness of the daz/y variation of the temperature of 
the surface of the sea, which are shown by the C/a//enger observa- 
tions, as discussed by Mr. Buchan, not to exceed 1° F., as com- 
pared with the large daily variation on land, there result directly 
the land and sea breezes with all their beneficial consequences. 
Similarly from the small yearZy variation of the temperature of 
the sea, as compared with the very large variation of the tem- 
perature of the land surfaces of the globe, result those great 
annual changes of the prevailing winds—the most important of 
which, with respect to widespread climatic effects, is the summer 
monsoon of the Europeo-Asiatic continent. 
But the most important, as well as the most direct, effect of 
the unequal distribution of tem: erature over the surfaces of the 
oceans and continents, is an unequal distribution of atmospheric 
pressure varying more or less with season. On the one hand, 
in a particular season we see a portion of the earth’s surface with 
atmospheric pressure much less than in surrounding regions, and 
as long as the low pressure is maintained the winds from the 
regions all around continue to blow inwards upon it, bearing 
with them the temperatures and humidities of the regions from 
which they have come. On the other hand there are other 
parts of the earth’s surface with atmospheric pressure much 
higher than in adjoining regions, and, as this state of things 
continues with little variation throughout the year, the winds 
blow out in all directions towards surrounding regions. Of this 
two illustrations may be given. 
During winter months atmospheric pressure is much less in the 
North Atlantic about Iceland than it is all round, and towards 
this area of low pressure the winds from the surrounding 
continents blow vorticosely, thus determining the winter climates 
of the more important countries cf the world. Over Canada 
and the United States the winds are north and north-westerly, 
by which the rigours of winter are intensified ; but in Western 
Europe the prevailing winds are south-westerly, and, as these 
winds bring with them the warmth and moisture of the Atlantic, 
the winter climates of Western Europe contrast strongly, latitude 
for latitude, with those of the eastern states of America. + 
Again, pressure is higher in the Atlantic between the north 
of Africa and America than it is all round, and out of this 
anticyclonic area of high pressure observations show that the 
winds blow in all directions towards surrounding regions where 
pressure is less. To the westward of North Africa the prevailing 
winds are northerly and north-westerly, but on the south side of 
this anticyclonic region the winds are easterly, and on the west 
the winds are southerly. 
Owing to these very different winds, and the oceanic currents 
to which they give rise, the temperature of the sea is much 
higher off the coasts of Florida than it is off the coasts of Africa 
in the same latitudes. The effect of these differences is re- 
cognisable in the distribution of marine life and coral reefs, and, 
consequently, of the deposits at the bottom of the sea. 
Since over this anticyclonic area, and similar ones in the 
South Atlantic, North Pacific, and in a less marked degree in 
the South Pacific, atmospheric pressure remains high throughout 
the year, notwithstanding the outflow of wind all around from them, 
it follows that aérial upper currents must flow towards these high 
the air through their central portions. Now, as observations 
show that in such circumstances the sky is clear, the air dry, the 
rainfall small, and the evaporation large, it follows that over 
these parts of the great oceans, where atmospheric pressure is 
higher than all around, the rainfall is very far from being suffi- 
cient inamount to make good the loss arising from evaporation— 
a consideration which has important bearings on the difficult 
question of oceanic circulation. 
As in these anticyclonic regions in the great oceans there is 
opened up a direct communication between the upper regions of 
the atmosphere and the surface of the sea, hy means of the 
descending aérial currents, it is interesting to ask whether this 
fact may not have some connection with the volcanic and 
cosmic dust found in the same regions in the deep-sea deposits ; 
especially is this interesting in connection with recent specula- 
tions as to the presence of these substances in the higher regions 
of the atmosphere. 
In thus indicating the positions of the high-pressure areas, 
and of the winds that blow out from and around them over the 
great oceans, we have at the same time traced the courses of the 
great oceanic currents and the positions of the Sargasso seas, 
for the winds everywhere determine and control the movements 
of the surface waters. 
The moisture taken up from the sea surface by the winds— 
leaving the water salter than before—is borne to the land and 
condensed on the mountain-slopes. Eventually this water 
gathers off the land, passes by rivulet, stream, and river down 
again to the ocean, bearing along with it a burden of earthy 
matters in solution. In this manner the ocean has most probably 
become salt in the course of ages. The water of the ocean now 
contains, it is almost certain, a portion of every element in solu- 
tion. Many of these are present in exceedingly minute traces. 
They are detected either in the sea water or the evaporated- 
down residue by spectrum analysis; in the copper of ships’ 
bottoms, which have withdrawn them by chemical decompo- 
sition ; or, again, in the ashes of sea-weeds and marine animals, 
which, during life, exert a selective influence upon the surround- 
ing water. 
(A diagram was exhibited showing the average composition of 
sea salt.) The individual salts present in sea water are, of course, 
constantly interchanging their metals and acid radicals, so that it 
is impossible to say authoritatively what is the precise amount of 
the respective chlorides and sulphates of sodium, potassium, cal- 
cium, and magnesium actually present. But it has been shown 
by hundreds of laborious and most delicate experiments that the 
actual ratio of acids and bases in sea salts—that is, the ratio of 
the constituents of sea salts—zs comstant in waters from all 
depths, with one very significant exception—that of lime—which 
is present in slightly greater proportion in deep water. 
The total amount of dissolved salts in the ocean would, it is 
calculated, if extracted, form a pavement 170 feet thick over 
the entire sea-bed, and of this amount 14 inches would be com- 
posed of pure carbon, chiefly present as carbonic acid in the 
carbonates. 
On account of the constancy in its composition the determina- 
tion of any one of the constituents of sea salt—chlorine, for 
instance—gives the datum for calculating the salinity—that is, 
the proportion of total salts to the water in which they are 
dissolved ; though determinations of this nature are more con- 
veniently made by observations of density by means of the 
hydrometer. (A map was exhibited on which Mr. Buchanan has 
shown the results of his laborious investigations in this direction. } 
An examination of this shows that the surface water of the ocean 
is freshest—that is, contains the least salt—at the poles and in the 
equatorial belt of calms. In the east of the Indian Ocean a 
change of the monsoons brings about a great change in the 
salinity of the surface water. ‘The centres of the great systems 
of oceanic currents produced by the trade winds are the areas of 
highest salinity in the open ocean ; yet here the water is not so 
salt as in some enclosed seas situated in areas of great evapora- 
tion, as the Mediterranean, and especially the Red Sea and 
Persian Gulf, where the saltest water is found and where a 
regular circulation is kept up by the outward flow of the denser 
water. The salinity of the deeper waters is considerably below 
the average at the surface in the open ocean, especially in the 
Atlantic. 
In the equatorial regions the surface water of the ocean has 
occasionally a temperature of 85° or 86° F., and the norinal 
temperature in tropical and sub-tropical regions ranges from 60° 
pressure regions accompanied by a slow downward movement of | to 80°. This warm water is, however, a relatively thin stratum 
