22 



where a is the projection on the sea surface, and is the geographical 

 latitude. Substituting this new value for -tt we have 



2co -t7 sm ^ 



(d) 



da 



hut^=A'B'D"C' = {co-Ci),L where 



Co = velocity per second in a given horizontal plane. 

 Ci = velocity per second in another horizontal plane. 

 L = distance between stations. 



Substituting in (d) for the new value -tt we have 



2a) sin (t> (Co — c^Z, (e) 



STA^XIOM ^ 



Fig. 6.— Lines AA' and BB' represent the velocity of the surface current, or co, per unit T; CC 

 and DD' indicate the velocity of the current at a greater depth, or ci. The difference in the 

 velocity of the two movements is equal to D" B', or co— ci. The movement is assumed to be 

 normal to the vertical plane ABDC, which is passed through the two stations A and B. Area 

 C" D" B' A', indicated by the symbol <r, represents the difference in the change of areas per 

 change of T, projected on the sea surface and developed by the progression of the two lines AB 

 and CD with the respective velocities co and c>. 



Thus by (e) we are furnished with an expression for the effect of 

 terrestial rotation in terms of the latitude; the distance between 

 stations; and the difference in velocity of the current between any 

 two levels. It is easy to see that if we are able to find some point along 

 the verticals where zero velocity prevails, then we have a means of 

 expressing the real velocity. It is customary to extend the investi- 

 gations to depths where it is believed motionless water lies, and then 

 Ci = 0, and Co is the true velocity on the surface. (See p. 13 regarding 

 the obliquity of isobaric surfaces.) 



