804 
winds. With easterly winds (N. N. E., to 
8. S. E.), the water flows into the North 
Sea through all these channels, and with 
westerly winds (N. N. W., to W.S. W.), the 
current reverses from the North Sea into 
the Baltic. The difference between the 
quantity of water which, within a specified 
time, is carried out of the Baltic and into 
it, indicates the quantity which has been 
definitely removed from the Baltic within 
the time, and furnishes an indication of the 
force of the Baltic current; hence the cur- 
rent has no specific existence but only a 
differential one. 
Assuming? to be the time during a year 
when outgoing currents prevail, and v the 
mean velocity, likewise ¢’ the length of time 
for inflowing currents during the period, 
and v’ their mean velocity, the mean veloc- 
ity of the Baltic current will be given by 
the formula 
Dr. Cronander has calculated this velocity 
for the sound for two decades, 1850-59 and 
1864-73, and obtained for the first period 
y= 1.204, v = 1.304 and V = 0.257 and for 
the second period v = 1.153, v’ = 1.230 and 
V=0.210 (velocities in knots per hour). 
It will be noticed that the mean current 
from the North Sea is stronger than that 
from the Baltic, and since, in spite of this, 
more water is conveyed from the Baltic than 
into it, the outgoing current must be of 
greater duration. Similar calculations have 
also been made for the Great Belt, but only 
for the term of a few years; they show that 
in some years more water flows through 
this passage into the Baltic than oud of it. 
These measures of velocity apply to the 
surface only, and now the important ques- 
tion comes up, to what depth is the current 
propagated and what is the law of decrease 
with increasing depth? 
Currents which are caused directly by the 
SCIENCE. 
[N. S. Von. X. No. 257. 
winds, denominated ‘ drift currents,’ show a 
very rapid decrease of velocity with increase 
of depth in consequence of friction, as indi- 
cated in the adjoining diagram (A); in 
rivers, where the velocity is due to the dif- 
ference of level, the decrease of velocity 
with the depth is very gradual as shown by 
the accompanying diagram (B). The dif- 
ference in the shape of these diagrams is 
so apparent that nobody can make a mistake 
in deciding whether a given current belongs 
to one or the other of the two classes. 
Now Dr. Cronander found that wherever 
he measured the currents under the surface, 
whether in the Baltic, the Sound, the Belt, 
or the Cattegat, the observations always 
pointed to differences of level as the cause, 
and against a supposition of a direct effect 
of the winds. This is then another im- 
portant conclusion which Dr. Cronander 
reached, viz.: that although the currents of 
the Baltic appear to obey the winds, the 
winds are not the immediate cause, but the 
difference of level created by preceding 
winds. ‘To illustrate : Supposing a strong 
westerly wind to have been blowing over 
the Baltic, it will produce an accumulation 
of water in the eastern and northern part, 
and a corresponding depression of level in 
the western part. As soon as these west- 
erly winds are replaced by easterly ones, the 
pent up waters will flow in an easterly di- 
rection, more in consequence of reaction 
against the preceding westerly winds, than 
in obedience to existing easterly winds. It 
must be observed that the greatest veloci- 
ties were not generally found at the surface. 
Thus, in the Sound the average velocity of 
the outgoing current, 3.55 decimeters per 
second, was found at the surface; that of 
the ingoing current, 3.66 decimeters at the 
depth of 1 fathom. For the Great Belt, the 
corresponding figures were 3.81 decimeters 
at 4 fathoms for the outgoing, and 3.78 at 
6 fathoms for the ingoing current. The 
shift of greatest velocity from a surface to a 
