and Distribution of the Atmosphere. 201 



To prevent misconception, I should premise that the problem 

 which the theory professes to solve is a broad one. It professes 

 to describe and account for the grand currents of atmospheric 

 circulation over the earth's surface as a whole, and the distribu- 

 tion of the atmosphere in greater or less quantity over the dif- 

 ferent parallels of latitude, without taking account of local pecu- 

 liarities depending on the irregular distribution of land and 

 water. Their discussion constitutes a separate subject of very 

 great importance ; but as it is easier to conquer difficulties sepa- 

 rately than combined, it seems reasonable to begin by neglect- 

 ing these local complications and treating the earth as a solid of 

 revolution with a uniform surface, especially as we have, in 

 large portions of the existing oceans, a good approximation to 

 these hypothetical conditions, and an opportunity of comparing 

 theory with observation. 



The actual state of things over these parts of the ocean 

 may be described by saying that : — 



I. As regards wind, there is an equatorial belt of calms, then, 

 on each side of this, a belt some 20° wide covered by the trade- 

 winds, which blow from the east, and at the same time towards 

 the equator, then another calm-belt near the tropic, and then a 

 region, extending as far towards the poles as observations go, 

 over which the prevailing winds are from the west and at the 

 same time towards the pole. 



II. As regards quantity of air, the barometer is low in the 

 equatorial calm-belt, from which it gradually rises across the 

 trade-wind region to the tropical calm-belt, where the pressure 

 is greater than on any other part of the earth ; and from this 

 latitude, as far as observations extend, there is a regular fall 

 towards a minimum at each pole, the pressures actually ob- 

 served in very high latitudes, especially near the South Pole, 

 being very much lower than in any other part of the earth at 

 sea-level. 



The theory which I am advocating asserts that these two 

 sets of phenomena stand in the relation of cause and effect. It 

 asserts that the distribution of barometric pressure (in other 

 words, the distribution of the air in greater or less quantity 

 over the different parallels of latitude) is mainly due to the 

 easterly and westerly components of the winds. 



It regards the surface of the earth as a surface of equili- 

 brium under the joint action of gravity and the centrifugal 

 force of the earth's rotation. But in the case of a body moving 

 west or east relative to the earth's surface, this equilibrium 

 no longer exists, because the body has greater or less centri- 

 fugal force than would be required for equilibrium. West 

 winds consist of air revolving faster than the earth, and there- 



