240 LIGHT SCIENCE FOR LEISURE HOURS. 



more intense action, as its maximum effect is limited 

 to a much smaller area than that of the maximum of 

 equatorial heat. The action of the trade and counter- 

 trade winds, in like manner, cannot be ignored ; and 

 henceforward the question of ocean-currents will have 

 to be considered under a twofold point of view.' 



It appears to me that not only is the equatorial or 

 rather tropical action much wider in range, but it is 

 also more intense than the polar action. For, let us 

 consider what happens during the heat of the day over 

 the tropical Atlantic. Here, over an area enormously 

 exceeding the whole arctic basin (we are considering, 

 be it understood, only the northern part of the system 

 of circulation) a process of evaporation is taking place 

 at so rapid a rate as to furnish almost the whole of 

 that rain-supply whence the rivers of Europe and North 

 America (east of the Eocky Mountains) take their 

 origin. There is thus produced a continual defect of 

 pressure, not merely along an equatorial strip, but so 

 far as 20 or even 30 degrees of north latitude, while 

 the downfall of rain which, taking one part with an- 

 other of the temperate and sub-arctic Atlantic, may be 

 regarded as incessant, continually adds to the pressure 

 in these last-mentioned regions. That on the whole 

 there must result a most effective excess of pressure 

 over the temperate zone of the Atlantic, as compared 

 with the tropical and equatorial portion, seems to me 

 indisputable. Now, if we compare this with the effects 

 of refrigeration over the relatively insignificant arctic 

 area, which as I have said has to supply the North 



