108 
MR. H. X. DIGKSOX OX THE CIRCULATIOX OF THE 
dimiimtiou in the number 'and energy of the cyclones following the Icelaiid low- 
pres.siire belt. The effect is to weaken t]ie drifts in the higher latitudes, and to 
strengthen them in the south, the latter l)eing sup})orted hy the fall of pressure over 
the North American continent. The fall of pressure over Europe, and the tendency 
of the Atlantic anticyclone to project north-eastwards, causes steady westerly winds 
to })revail over a broad belt in the widest part of the North Atlantic, and the drifts 
accordingly set eastward against the land in much greater volume, and much further 
north, than in winter. Hence there is a e'reat increase in the relief current inovino- 
northward ; this current is known to extend down to 300 fathoms at the Wyville- 
Thomson liidge, and to penetrate far into the Arctic regions (45). 
The enormous cpiantities of warm Atlantic water sent north by this current are 
much more effective than the seasonal warmino- of the air in melting’ the ice of the 
Arctic seas, and the southward movement of the Polar water is apparent in July and 
August. Tlie light variable winds prevalent at that season do not induce any marked 
drift of these waters. 
During autumn the transition phases of spring are reversed : the coast currents 
again become weaker through the changes of wind force and direction due to the 
O o O 
shrinkage of the Atlantic anticyclone, and the drift system is re-established. The 
first result is to spread the Polar waters eastward over the Atlantic area, where they 
are more or less rapidly absorbed by mixture with underlying water; but the mixed 
Avaters may partly or wholly cover over the weakened and retreating coast current 
so well marked in the summer season. 
Tlie additions made to our knowledge of the warm northerly currents in the lijgher 
latitudes of the Atlantic by recent expeditions have been fully summarised and 
discussed by Petterssox since this inve.stigation was begun (46), and the conclusions 
ariived at with ingard to them are fully supported by the extended surface observa¬ 
tions in the lower latitudes, d’he great development of these streams is to be 
accounted for by the transfer of the warm salt waters, sent up along the American 
coast by the Gidf Stream, as surface drifts to the south-western coasts of Europe, 
where they are Ijanked up, stored as it were in a vast resei-voir, fi'oni which they escape 
northwards, southwards, and downwards, filling the whole basin of tlie eastern and 
nortli-eastern Atlantic, and overflowing as northward streams wherever the form ot 
the basin makes it possilJe. Tliese northward currents are })ermanent, hut they 
suffer variations corresponding t(j the changes in the rate at which the drift-water is 
accumulated, and in their more remote branches they have a surface circulation 
superposed vi})ou them—a thermal circulation in the late summer and early autumn, 
and a drift circulation in late autumn and winter. 
Tlie general circulation of the North Atlantic is therefore the result of a large 
number of factors, each of which is subject to wide variation. From a consideration 
of the mean result in its relation to the mean atmosplieric circulation, it appears that 
the oceanic circulafion is directly couti-olled by the winds; the form, position, and 
