and Distribution of the Atmosphere. 203 



treating of this subject, express themselves so as to convey the 

 idea that such a body tends to preserve its absolute eastward 

 velocity constant* ; but, as Mr. Ferrel has remarked, this is an 

 understatement of the fact ; it is not velocity, but moment of 

 momentum that tends to remain constant, and absolute eastward 

 velocity tends to vary inversely as distance from axis. 



The easting of the trade-winds is thus clearly accounted for. 

 But the principles thus far adduced would give us in the northern 

 hemisphere only north-west and north-east winds at the surface 

 of the earth, w T ith return-currents from the south above. How is 

 the prevalence of south-west winds at the surface of the earth in 

 the north temperate zone, and of north-west winds in the south 

 temperate zone, to be accounted for ? The account which I be- 

 lieve to be correct, and which in its essential features is due to 

 Professor James Thomson, is as follows :— - 



If any stratum of air have less than the average eastward or 

 westward velocity (relative to the earth) which prevails through 

 the strata above it, it will not be able to resist the differential 

 pressure from or towards the equator which their motion pro- 

 duces. For this reason the lowest stratum of air, having its 

 velocity relative to the earth kept down by friction, generally 

 moves from the tropical belts of high barometer to the regions 

 of low barometer at the poles and equator. This is the origin 

 of the prevalent winds of the two temperate zones, which must 

 be regarded as constituting under-currents towards the poles, 

 beneath a topmost current, also towards the pole, and a middle 

 return-current. 



Between the tropics, on the other hand, the motion thus ge- 

 nerated in the lowest stratum of air coincides with the motion 

 due to difference of temperature; this is probably the reason 

 why the trade-winds are more constant than the winds of the 

 temperate zones. 



The easting of winds blowing towards the equator, and the 

 westing of winds blowing towards the pole, may be summed up 

 in the rule that the wind tends to swerve to its right in the 

 northern hemisphere and to its left in the southern : that is to 

 say, in the northern hemisphere, if not constrained by differen- 

 tial pressure, it will blow along a curve concave to its right hand ; 

 and if it is not allowed thus to swerve, but constrained to keep 

 a direct course, it exerts greater pressure against the air on its 

 right hand than against the air on its left. But this tendency 

 is not peculiar to air which is moving in a northward or south- 

 ward direction. We have already pointed out that air which is 

 moving from west to east over the earth possesses an excess of 

 centrifugal force, in virtue of which it tends to deviate towards 

 * This mistake occurs in Hadley's original paper. 



