STRUCTURE AND SEA-BIRDS OF THE N. ATLANTIC 5 



from left to right. The points travelling with the greatest velocity will 

 be those on the equator, and the two points represented by the Poles 

 will travel with no velocity relative to the earth's axis. 



In general terms it is true that, as the earth rotates, its atmos- 

 phere rotates with it. However, there is a certain effect due to inertia 

 or drag; and this effect, obviously, is greatest at the equator, where 

 the surface velocity is greatest. The efTect operates on all objects 

 but can put only liquids and gases into a dynamic state. Upon 

 these Corioli's force — the deflecting force of the earth's rotation — 

 acts in a simple manner. It sets them in motion in a direction which, 

 at the equator, is opposite that of the rotation of the earth. Thus if we 

 examine a map of the prevailing winds and ocean currents of the 

 world, we find pronounced positive east-to-west movements in all 

 equatorial regions. The liquids and gases thus displaced circulate into 

 the temperature regions and perform return movements in the higher 

 latitudes where the Corioli's force is less. Consequently, in the northern 

 hemisphere water and wind currents tend to turn right-handed, 

 whereas in the southern hemisphere they turn left-handed. (Exceptions 

 to this rule are mostly found in minor seas, where the impact of the 

 currents upon coasts may cause contra-rotation.) The main clockwise 

 movement of the northern hemisphere wind and currents is very 

 obvious. (See the map on a back end-paper.) 



The Atlantic equatorial current can be traced from the African 

 coast south of the equator westwards as far as the sea reaches. Approach- 

 ing the coast of Brazil it attains a remarkable speed. It sets past the 

 isolated oceanic island of Ascension so that even in calm weather it 

 leaves a wake of turbulence which must make that island unusually 

 visible from far off by its numerous bird inhabitants. 



Just north of the equator the lonely St. Paul rocks, which represent 

 the pinnacles of a submerged, steep-sided mountain over thirteen 

 thousand feet high, face the full strength of the great equatorial current, 

 especially in August, when the associated south-east trades are blowing 

 their hardest. During the cruise of the Challenger in i860 H. N. Moseley 

 saw the great ocean current "rushing past the rocks like a mill race." 

 A ship's boat was quite unable to pull against the stream. 



The equatorial current divides when it impinges on the corner 

 of Brazil at Cape Sao Roque. The northern element — the Guiana 

 coast current flows past the mouth of the Amazon with sufficient 

 rapidity to displace the outgoing silt 100 miles or more in a northerly 

 direction; and it continues steadily past the mouth of the Orinoco 



