OCEAN TEMPERATURES AND CURRENTS. 24I 



true effect upon the currents. Secondly, neither are 

 the temperature differences able to do what is ascribed 

 to them. It is very clearly shown by Croll that their 

 effective power has been overestimated. He finds an 

 unexpectedly small value for the velocity of the current 

 induced in this way. Further, the theory demands much 

 lararer and more extensive currents than we know to 

 exist. 



II. Those who advocate the friction of the wi^ids as 

 a cause. This side has been taken by Herschel, Frank- 

 lin, Croll, Zoppritz, and Haughton. Its best defence is 

 in the mathematical work of Zoppritz. This school 

 claims that the force of the wind applied to the surface 

 of the water constantly for a given period will transmit 

 some of its velocity by means of the motion of the 

 molecules upon one another. Taking the coefficient 

 of the friction of water as 0.0144, it would take about 

 240 years for this velocity to be transmitted to a 

 depth of 400 feet ; of course, if the velocity varies on 

 the surface, this time will vary, but we have ages to 

 draw upon. 



Professor Ferrel has raised the only objection to this 

 theory which seems valid at first sight, namely, that the 

 moment of velocity of the sea current is greater than 

 that of the wind, and hence we are trying to prove the 

 greater by the less. He has apparently not considered 

 the constancy of the winds and the time element in- 

 volved. Many instances could be given of the effect of 

 the wind upon the water, such as the massing of the 

 waters of some of the Swiss lakes at one end of their 

 basins, -after the winds have blown constantly in that 

 direction for several days, The most marked example 



