64 
MPv. H. N. DICKSON ON THE CIRCULATION OF THE 
ments of charts showing the seasonal changes of mean surface temperature, changes 
which are in part affected, or effected, hy variations in the currents, and a few 
“ current charts,” showing tlie mean residt of numl^ers of surface current ohservations 
mafle at different seasons of the year (6). In these cases, however, it was of course 
necessary to ignore the possibility of irregular or long-period changes, trusting to a 
sufficiency of observations to give a})proximate seasonal values from the means. 
It seems scarcely necessary to attempt to account for the gradual recognition, 
during the past decade, of the existence of extensive changes in the circulation of 
oceanic waters at different times. The discoveries of the “Challenger” Expedition 
with regard to the geographical distribution of maiiiie organisms raised innumerable 
(juestions which demanded fuller knowledge of the physical and chemical conditions 
than coidd be obtained hy merely extending and continuing ol)servational work 
along the old lines. The development of a general mathematical theory of atmo- 
S})heric circidation which agreed with the residts of observation in the main outlines, 
1)ut presented many local anomalies, required moi'e detailed study of the physical 
conditions of parts of tlie earth’s surface, and es})ecially over the sea, for their eluci¬ 
dation. Lastly, the necessity for the regulation of some of the great fisliing industries 
was becoming increasingly urgent, and it was more and more evident that any such 
regulation must l)e Imsed on full scientific knowledge of the physical and chemical 
conditions upon which, directly or indirectly, tlie positions of the great fishing 
grounds had been shown to depend. All tliese influences worked in the same direc¬ 
tion, and the economic importance of the fishery question strengthened the hands of 
the societies or government departments upon which the expense of further investiga¬ 
tion must fall. 
The first investigation carried out with the requisite detail and accuracy of method 
was that undertaken l)y Professor F. L. Ekman, who directed the Swedish explora¬ 
tion of the Baltic in the year 1877 (7); and it is to tlie subsequent labours of Swedish 
hydrographers, and especially of Professor Otto Petterssox, of Stockholm, that the 
development of the modei-n methods of research are very largely due. Put shortlv, 
the outstanding feature may lie said to he the application of the idea of the syno]itic 
chart—the survey of the part of the ocean under investigation in such a manner that 
the physical or chemical conditions in its waters are known at successive instants of 
time, at intervals sufficiently short to allow of the changes being continuously traced. 
While allowing a full measure of credit to the Swedish hydrographers in this respect, 
it is necessary to notice that similar methods wei'e develo])ed independentlv by the 
officers of the United States Coast and Geodetic Survey and hy Professor W. 
Ltbeey, Jun. (8), in their work on the Gulf Stream, and hy Mile and Mueeay (9) 
in the Clyde Sea Area, and on various lakes and fjoi-ds in Scotland. 
As the wmrk of the Swedish hydrographers in the Baltic and Skagerak progressed, 
they were driven further and further seaward in their inquiries, until, in 1890, 
Pettersson and G. Ekman (10) began a systematic investigation of the waters of the 
