CAXADIAX FlsHEh'IES EXPEDITIOX, l'Jl.',-lo 



255 



deformation of the diagram takes place, and by calculating a and / for the different 

 parts of the current, we can immediately ascertain the magnitude of the force acting 

 upon each ccm. of the water by means of the wind. If, however, the wind be blowing 

 against the current, then the diagram of velocity will be strongly curved back at the 

 surface, vide fig. 40 c. Here the retarding force of the friction is very great, whereas 

 at greater depths it retains its normal value. In this case likewise we may, by divid- 

 ing up the diagram and finding a and / for each portion, ascertain the magnitude of 

 the effect produced by the wind at ditforeut depths. 



XL 



Fig. 40. Diagram of velocity fur a surface current in 

 calm weather and with wind in same and the oppo- 

 site direction. 



In dealing thus with the diagram of velocity, due regard should be had to the 

 imperfection of the methods of measurement. Thus the small irregularities in the 

 upper part of the diagram fig. 36 cannot be considered as reflecting actual conditions, 

 but are rather due, partly to the inaccuracy of the instruments employed, partly to the 

 fact that the measurement.s were not taken at exactly the same time for the different 

 depths. If these irregularities be levelled down, however, the parabolic form is quite 

 distinct. We find then, that the water down to 10 metres depth is sucked in under the 

 ice with great force, and that beneath this surface current, two outward currents of 

 inferior velocity are found at 13 and 35 metres depth. The water between these last- 

 named currents is drawn along by tliem. 



The foregoing examples should suffice to give an idea as to the great value of the 

 diagram of velocity in calculating the forces acting upon a current, in considering the 

 disturbances to which it is subjected, and in studying its nature and composition 

 generally. Such diagrams 'should therefore be more widely employed in marine 

 investigations than has hitherto been the case. 



10.— ON THE CAUSE AND THE EFFECTS OF OCEAX CURREXTS. 



The causes which give rise to currents in the sea are either external forces, such 

 as the action of the wind, or physical changes in the sea-water itself, occasioning an 

 alteration of its specific gravity. We have thus two distinct categories of ocean cur- 

 rents, differing widely in character, and it is important to have a clear understanding 

 of these. 



If the surface water in a given area be subjected to physical change tending to 

 increase its specific gravity, as for instance by cooling or evaporation, it will com- 

 mence to sink, and the lighter surface water surrounding will flow in from all sides. 

 And owing to the rotation of the earth, a cyclonic movement then sets in about the 

 sinking centre. We have thus a movement in towards the centre, and at the same 

 time a cyclonic movement round that centre, as shown in fig. 41 a. If, again, a cyclonic 

 wind, by friction upon the surface of the sea, should set the surface water in cyclonic 

 circulation, then the current in question will, owing to the earth's rotation, veer off 



