Currents 59 
surface, or the amount of rainfall, melting ice, or the inflow of 
large rivers. 
The prevailing winds of certain latitudes also help to build 
up ocean currents. 
DEFLECTION OF CURRENTS BY EARTH’S ROTATION 
A glance at the Chart of Ocean Currents shows that in- 
stead of moving in straight lines the ocean currents are de- 
flected clockwise in the Northern Hemisphere, and counter- 
clockwise in the Southern Hemisphere. This deflection is due 
to the rotation of the earth. How does it arise? 
The earth rotates to the East, imparting to every particle of 
matter of land and sea a fixed angular momentum. A law of 
dynamics (the physics of moving bodies) states that the 
angular momentum of a particle must always be a constant 
amount. 
Inasmuch as angular momentum is made up of both the 
angular velocity and the distance of the particle from the 
axis of rotation, a change in one of these two quantities neces- 
sitates a corresponding change, but in the opposite direction, 
of the other. As a body of water moves North from the Equa- 
tor, as in the Gulf Stream, its distance from the earth’s axis of 
rotation is decreased. As a result its angular velocity must in- 
crease. This amounts to a movement East as well as North, 
which is the observed fact for the Gulf Stream. By a similar 
analysis the deflections of all the other ocean currents may be 
accounted for. 
HOW ARE CURRENTS MEASURED? 
An obvious way of measuring currents is to set sealed 
bottles containing a message afloat in the ocean, and to hope 
that they will be picked up eventually and returned. From a 
