180 Mr. J. Croll on Ocean- currents in relation to the 



the greater the tides ; and a few degrees north or south, where 

 it varies to near its average of 3 hours, they are recorded as 

 " irregular" or " barely sensible." A similar contrast occurs be- 

 tween the two sides of Madagascar. On its east coast, where, 

 from a wide ocean, the sea-wave and earth-wave arrive together, 

 the tides never anywhere equal 3 feet (or a third of Newton's 

 and Airy's calculated average for spring tides). But on the 

 landward side they are " considerable," and larger the nearer 17° 

 (that is, the later their establishment, till at the above latitude 

 it is retarded about 6 hours). 



In short, I conclude the tropical tides are only differential, 

 and become large only where the hour of high water is so re- 

 tarded by local circumstances as to correspond nearly to low earth. 

 But to high latitudes the earth-tides of course do not sensibly 

 reach, thus leaving the derived sea-tide to acquire the magni- 

 tudes familiar to us. A good test of this view might be found 

 in equatorial estuaries long enough to have several hours' differ- 

 ence of establishment — especially the Amazon, the only river I 

 find alleged to have several tides within it at once. These 

 ought, instead of the character of progressive waves, to show 

 rather that of stationary or see-saw ones, their summits rising 

 at fixed places, and having between every two a place of mini- 

 mum tide 5 namely, where the water establishment is the same 

 that I suppose the ground to have everywhere. 



London, January 8, 1870. 



XXIV. On Ocean-currents. By James Cuoll, of the Geological 

 Survey of Scotland*. 



Part II. 



Ocean- currents in relation to the Physical Theory of Secular 



Changes of Climate, 



JTIEFLECTION of Ocean- currents the chief cause of Secular 

 Changes of Climate. — The enormous extent to which the 

 thermal condition of the globe is affected by ocean-currents seems 

 to cast new light on the mystery of geological climate. What, 

 for example, would be the condition of Europe were the Gulf- 

 stream stopped, and the Atlantic thus deprived of one-fifth of 

 the absolute amount of heat which it is now receiving above what 

 it has in virtue of the temperature of space ? If the results just 

 arrived at be anything like correct, the stoppage of the stream 

 would lower the temperature of Northern Europe to an extent 

 that would produce on it a condition of climate as severe as that 



* Communicated by the Author. 



