yuly 15, 1886] 
NATURE 241 
Whether the surface of a bog becomes covered with 
forest or not, whether the peat grows or not, whether 
during the rising the erosion is strong enough to hollow 
out the shore-line, or the carrying power of the river is 
great enough for the formation of terraces, whether the 
edge of the inland ice recedes or !advances, whether 
a deposit of clay or marl is to be found in a certain place 
near the shore, or whether chalk only is left—ay e7- 
ttrely depend on small variations in the climate, as the 
conditions will alter as soon as a certain point is reached. 
The periodical changes dealt with here were therefore not 
great ; but as they acted simultaneously, and in the same 
direction, over whole climatic areas, it must be generally- 
acting forces which caused the same, and not variations 
in local conditions. 
The theory advanced here proves thus that the climate 
is at all times subjected to periodical changes, the 
duration of which may be measured in thousands of 
years, and which act in the same direction within the 
same climatic area, which for one period are not important, 
but which, as the alternation of the strata is often re- 
markably regular, see fo return after the lapse of a fixed 
cycle of years. 
It is obvious that periodical changes in the strength of 
the ocean currents will cause corresponding changes in 
the climate of the adjacent continents. Thus, for in- 
stance, if the warm North Atlantic current, to which 
North Europe owes its climate, which is mild compared 
with its latitude, should increase in strength, the climate 
there would doubtless become still milder. Our shell- 
banks show that such changes in the temperature of the 
sea have accompanied climatic variations. We are, 
therefore, compelled to ask, What is the force which 
causes this warm sea-current to flow northwards, and 
may we assume that there is some natural cause effecting 
periodical changes in the intensity cf this force? The 
question being one as to a climatic period, we must 
examine the great laws which govern the climate. We 
must, of course, leave all temporary disturbances of the 
air out of consideration, and only pay attention to the 
great and simple laws which are revealed by the synoptic 
charts of the average distribution of the aérial pressure 
at various seasons. These charts show us:—in the 
summer a low pressure over the heated continents, but 
generally a higher one over the cool oceans ; and in the 
winter a higher pressure over the cold continents, and a 
lower one over the oceans, which are warmer. 
In order to understand this varied distribution of 
pressure, we shall imagine an atmosphere which every- 
where has the same degree of heat and the same height. 
The warmer the air the more it expands, so that the 
height of the atmosphere will change if the temperature 
rises or falls. If we further assume that the air cools or be- 
comes more quickly heated in some places than others, the 
equilibrium will be disturbed. Over cold areas the height 
of the atmosphere will decrease. The surface of the at- 
mosphere should thus become uneven, and consequently, 
in the upper strata of the atmosphere air must flow from 
the warm regions into the cold ones, so that equilibrium 
be maintained. For this reason a greater mass of air 
will lie over cold regions, which have, therefore, a higher 
atmospheric pressure. But at the surface of the earth, 
too, the equilibrium will be disturbed, as a higher atmo- 
spheric pressure wiil drive the air from the cold to the 
warm regions. As long as the temperature of the air 
varies, movements will be created by the disturbed equi- 
librium, during which, therefore, air will flow from the 
cold to the warm regions along the surface of the earth, 
and vzce versdé inthe upper part of the atmosphere. In 
winter as well as summer the disturbances of the equi- 
librium of the atmosphere will proceed from the continents, 
because the latter are heated and cooled more intensely 
than the oceans. Over the ice-covered interior of Green- 
land the sun in the summer cannot create any low pressure, 
because all its heat is consumed in melting the snow. 
Even in the summer comparatively cold air and high 
pressure prevails over Greenland, and this is probably 
the cause of the atmosphere in the North Atlantic differ- 
ing from the above-mentioned law, inasmuch as _ this 
ocean has a low pressure even in summer. This low 
pressure, which lies generally near Iceland, is, however, 
more marked in winter. 
The air, according to the law of Buys Ballot, moves 
against the low pressures, so that in the Northern Hemi- 
sphere one has the low pressure a little in front to the 
left when turning the back to the wind. That is but a 
natural consequence of the rotation of the earth’s axis. 
At lower latitudes this action is more intense. Air, flowing 
from lower to higher latitudes, retains for a time its original 
speed of rotation, and will thereby deviate in the direction 
of the rotation of the earth’s axis, Ze. towards the east. 
And vice versa when the air flows from higher to lower 
latitudes. In this manner southerly winds become south- 
westerly, and northerly ones north-easterly. In fact, the 
low atmospheric pressure at Iceland draws the south-west 
winds up the North Atlantic, and as the cause prevails all 
the year round, the consequence is that south-west winds 
blow in this sea summer as well as winter. 
The opinion held by Croll, Zéppritz, &c., that winds 
are the chief cause of sea-currents, is now generally ac- 
cepted by savan/s. The winds set the surface of the sea 
in motion, and by frictional resistance the movement is 
conveyed to lower depths. It depends on the force and 
the duration of the wind how deep the action will have 
effect. The main current runs in the direction of the 
prevailing wind, and its speed is dependent on the average 
speed of the surface. Winds of short duration are only 
capable of changing the direction of the current on the 
surface, but through the predominance of such winds 
through thousands of years, great currents are created. 
Their strength may vary, 6u¢ their direction ts independent 
of temporary changes of the wind. For the upper system 
of currents, which alone affects the climate, and which 
reaches to a depth of a couple of hundred fathoms 
(Mohn), the average direction and force of the wind 
during the last great epoch are determinal. 
Such a great stream is the warm North Atlantic current. 
It softens the winter even at high latitudes. As the surface 
imparts heat to the air, the heat lost is replaced from 
lower depths, and as long as there isa store of heat below 
the sea will always yield heat to the air. 
The mild climate of Norway is, therefore, dependent on 
this warm current. It runs predominantly in a north- 
easterly direction, and thus it must, in consequence of the 
general laws for currents and winds, have run through 
untold ages, or as long as sea and land have been divided 
as at present. 
We will now see if the force which guides this current 
is periodically changeable. As we know, the orbit de- 
scribed by the earth round the sun is not circular but 
elliptical, so that the distance between the two bodies 
varies according to the seasons ; when there is winter in the 
Northern Hemisphere the earth is nearest to the sun, and 
the nearer the earth approaches the sun the quicker it 
travels, so that the winter in the north is shorter than the 
summer. ‘The difference is five days. In the Southern 
Hemisphere, on the other hand, the winter is five days 
longer than the summer. But these relations change 
through the precession of the equinoxes, the period having 
a mean duration of 21,000 years. Thus, 10,500 years ago 
the conditions were the reverse of what they are at present, 
and the same will be the case 10,500 years hence. The 
winter at the Northern Hemisphere will then fall when 
the sun is furthest from the earth, and last longer than the 
summer, and in the Southern Hemisphere the conditions 
will be the reverse. 
But the orbit of the earth is also subjected to periodical 
changes, inasmuch as it differs more from the circular 
