Mr. J. Croll on the Physical Cause of Ocean-currents. 189 



at the equator unless on some elevated mountain-region. On 

 the other hand, were the quantity of warm water which is being 

 transferred from the equator to be very much increased, the tem- 

 perature of intertropical latitudes might be so lowered as easily to_ 

 admit of temperate species of plants growing at the equator. A 

 lowering of the temperature at the equator some 20 or 30 degrees 

 is all that would be required ; and only a moderate increase in 

 the volume of the currents proceeding from the equator, taken in 

 connexion with the effects flowing from the following considera- 

 tions, might suffice to produce that result. During the Glacial 

 epoch, when the one hemisphere was under ice and the other 

 enjoying a warm and equable climate, the medial line between 

 the trades may have been shifted to almost the tropical line of 

 the warm hemisphere. Under such a condition of things the 

 warmest part would probably be somewhere about the tropic of 

 the warm hemisphere, and not, as now, at the equator ; for since 

 all, or nearly all, the surface-water of the equator would then be 

 impelled over to the warm hemisphere, the tropical regions of 

 that hemisphere would be receiving nearly double their present 

 amount of warm water. 



Again, as the equatorial current at this time would be shifted 

 towards the tropic of the warm hemisphere, the surface-water 

 would not, as at present, be flowing in equatorial regions parallel 

 to the equator, but obliquely across it from the cold to the warm 

 hemisphere. This of itself would tend greatly to lower the 

 temperature of the equator. 



It follows, therefore, as a necessary consequence, that during 

 the glacial epoch, when the one hemisphere was under snow and 

 ice and the other enjoying a warm and equable climate, the tem- 

 perature of the equator would be lower than at present. But 

 when the glaciated hemisphere (which we may assume to be the 

 northern) began to grow warmer and the climate of the southern 

 or warm hemisphere to get colder, the medial line of the trades 

 and the equatorial currents of the ocean also would begin to 

 move back from the southern tropic towards the equator. This 

 would cause the temperature of the equator to rise and to con- 

 tinue rising until the equatorial currents reached their normal 

 position. When the snow began to accumulate on the southern 

 hemisphere and to disappear on the northern, the medial line of 

 the trades and the equatorial currents of the ocean would then 

 begin to move towards the northern tropic as they had formerly 

 towards the southern. The temperature of the equator would 

 then again begin to sink, and continue to do so until the gla- 

 ciation of the southern hemisphere reached its maximum. This 

 oscillation of the line of maximum temperature to and fro across 



