DECEMBER 1, 1899. ] 
lower plane is satisfactorily explained by 
the interference of local winds with the 
direction of the surface currents. 
The existence of a heavier layer of salt 
water in the deeper part of the Baltic has 
been generally attributed to the existence 
of an undercurrent from the North Sea into 
the Baltic. Dr. Cronander found no traces 
of such an undercurrent in the shallow part 
of the Sound, where, at 5 fathoms depth, 
the ingoing and the outgoing currents each 
reached to the bottom. He compares the 
movement of the water here to that of two 
wedges, one of fresh water at the surface 
which has its base in the Baltic, and super- 
posed upon one of salt water which has its 
base in the North Sea; both wedges are 
driven asa whole backward and forward by 
the currents, with the effect that sometimes 
the whole Sound is nearly filled with fresh 
water and at other times with salt water. 
In the Great Belt the conditions were 
found to be somewhat different. With 
strong easterly or westerly winds, the out- 
going or ingoing currents reach to the 
bottom, but sometimes the brackish water 
from the Baltic combines with the salt 
water from the North Sea in the shallow 
part of the Belt, and constitutes a homo- 
geneous water-mass which is moved for- 
ward and backward. Again, sometimes a 
distinct bottom current is formed which 
moves in an opposite direction to the sur- 
face current. With low water in the Baltic, 
and an outgoing surface current, an in- 
‘going bottom current may arise; and 
with high water in the Baltic, and in- 
going surface current, an outgoing under- 
current may be called into existence. 
From these statements we conclude that 
the assumed undercurrent from the North 
Sea into the Baltic does not present the 
normal condition, but that undercurrents 
into and out of the Baltic may occasionally 
be called into existence in the Great Belt 
by an effort of water at the bottom, to re- 
SCIENCE. 
805 
store the hydrostatic equilibrium when it 
has been disturbed at the surface, and such 
a restoration at the surface is prevented by 
the winds. Hence, we may assume that the 
greater part of the salt water, which we find 
at the bottom of the Baltic, finds its way 
into it by surface currents, and that by rea- 
son of its weight and loss of heat by con- 
tact with the colder water, it settles to the 
bottom. 
In the northern Baltic, two alternating 
sets of currents were found; an outgoing 
one, coming from a N. N. E. direction with 
