298 transactions liverpool biological society. 



The Position of the Isotherms in the North Atlantic 

 during 1907-1913. 



The view has frequently been expressed in these 

 Reports that the movements of water in the Irish Sea, as 

 indicated by the salinity changes, serve as an index to the 

 seasonal changes in the North Atlantic. The detailed 

 observations of salinities in the open ocean necessary to 

 test the correctness of this view are not at present avail- 

 able, but it seemed possible that some help could be 

 obtained from the numerous temperature observations 

 which are constantly being made and summarised week 

 by week, in the form of isotherms of sea temperature, 

 and published on the Pilot Charts issued monthly by the 

 Meteorological Office. It is probable that the relative 

 position of these isotherms is to some extent dependent 

 upon the relative position of the warm salt water of 

 southern origin. 



It is not easy, however, to get a comprehensive view 

 of a large number of these isotherms, and recourse had to 

 be had to a graphical method of summarising them. 



The first method tried was that of making a diagram 

 for meridian 20° W. and meridian 50° W., in which the 

 positions of the isotherms for 50° and 60° were plotted in 

 a co-ordinate system, in which the abscissae were weeks, 

 and the ordinates the distances of the isotherms North of 

 Latitude 30°. These distances were obtained by direct 

 measurement from the charts. As the scale of the maps 

 on the earlier charts was different from that on the charts 

 as issued at present, they were reduced by a graphical 

 method to the scale of the charts as at present published 

 before being entered on the diagram. 



In this way a curve was obtained for each isotherm for 

 each year, each being drawn in ink of a different colour. 

 Even then, however, although there were often consider- 



