In the study which has been the subject of the foregoing pages we have always considered the motion of the air to be regulated by a distribution of pressure balanced by the rotation of the earth, except in regard to the surface layer and one other suggested exception when the momentum of the general westerly circulation was invoked. It should here be noted that by this limitation to what may perhaps be called “great circle motion,” we are considering almost exclusively the circulation above that half of the earth's surface which is north of the northern tropic and south of the southern one.