Ocean Temperatures along the West Coast of North America. 247 



impulse, first felt only at the surface, will gradually communicate itself 

 downwards, owing to the viscosity (internal friction) of the water, and 

 the lower strata to a successively greater and greater depth will thus 

 partake of the movement until it is finally shared by the whole mass, 

 the velocity diminishing as the depth increases. The rate, however, at 

 which the motion is communicated to the depths of the ocean is ex- 

 ceedingly slow. For example, Zoppritz (2) estimated that in a depth 

 of 4000 meters a surface current of a given velocity would require a 

 period of about 100000 years to transmit a velocity of one half this 

 amount to a point half way toward the bottom, and 240 years would 

 be needed for the layer at the depth of 100 meters to attain one half 

 the surface velocity. A period of 24 hours is required before a change 

 in the direction of the wind affects the water at a depth of 5 meters. 

 Similarly, when once established, these submarine currents exhibit a 

 corresponding reluctance to undergo any change in direction or intensity. 

 In general, according to Zoppritz, the time (t) in years required for 

 a given constant surface velocity to produce a velocity one half as 

 great at a depth of (x) meters is given approximately by the formula 



t = 0,024 x 2 . 



But observations show that perfectly steady winds do not exist, 

 even in Ihe region of the trades. The winds are constantly changing, 

 and the surface currents change with them. The lower strata of the 

 ocean, however are insensible to these changes, and at a considerable 

 distance below the surface, the waters of the ocean have probably a 

 slow but steady motion, the direction of the motion probably agreeing 

 closely with the resultant surface winds. 



James Page (3) summarises the conclusions based on Zoppritz's 

 theory as follows. We have, therefore, in the body of the sea two 

 distinct sets of currents; first, those at the immediate surface, which 

 move practically at the obedience of the surface winds, sometimes in 

 one direction and sometimes in another, second those of the lower 

 strata, which are constant in direction and velocity and represent the 

 aggregate effect of the winds that have blown for ages past. 



II. Review of the theories that have been proposed to account for the 

 cold-water belt along the west coast of North America. 



1. Ball (4) has discussed the dependence of the cold in-shore water 

 on the Japan Current somewhat as follows. The Japan Current is pro- 

 duced by the impinging of the Pacific North Equatorial Current on the 



