184 Mr Joseph John Murphy on the Circulation 



The upper and lower currents exercise friction on each 

 other, and so tend to destroy each other s momentum, and 

 the eastward momentum lost by the one must exactly equal 

 the westw^ard momentum lost by the other. But in addi- 

 tion to this the lower current must lose momentum by 

 friction against the earth's surface. Consequently, the west 

 component of the momentum of the upper current is much 

 greater than the east component of the momentum of the 

 lower one, and this preponderance of force causes the upper 

 currents to communicate their own west-erly motion to the 

 lower ones. At the equator the easterly motion of the 

 trade-winds must still prevail in a slight degree at all 

 heights in the atmosphere. At a very little way towards 

 the poles, the westerly motion begins in the upper stratum, 

 thence the upper stratum of westerly motion deepens, and 

 the lower one of easterlj' motion thins out, until about lat. 

 28° (taking the mean of both hemispheres) the former 

 appears at the earth's surface. From thence to the poles, 

 the air, in both its upper and its lower strata, constantly 

 circulates round the globe from west to east, constituting 

 what Maury calls the counter-trades. Every east wind in 

 higher latitudes is either merely a local phenomenon, or a 

 polar extension of the trade-winds. 



Professor Coffin, in one of the earlier volumes of the 

 Smithsonian Transactions, maintains, on the authority of 

 certain registers, that the prevalent direction of the wind 

 in very high latitudes is from the east. I do not understand 

 the reasoning by which he endeavours to account for this, 

 and I suspect it is a merely local or perhaps temporary 

 phenomenon. Sir James Eoss met nothing like it in high 

 southern latitudes, and, as we shall see further on, observa- 

 tions bearing on the great atmospheric currents are of more 

 importance when made in the southern than in the northern 

 hemisphere. We have every reason to believe that in the 

 northern hemisphere, during the summer half of the year at 

 least, the pole of greatest cold does not coincide with that 

 of rotation, and this would produce very complex and quite 

 incalculable motions. 



The principle of reaction makes it it impossible that the 

 winds can have any effect in either accelerating or retard- 



