GRAND CURRENTS OF ATMOSPHERIC CIRCULATION. 
G81 
as to what must be the general character of the motions of the air at various places in 
the Trade-Wincl Zone, both in the lower great current approaching to the Equatorial 
or Doldrum Belt,'" and in the upper great current departing from that Equatorial Belt 
and flowing aloft over the Trade-Wind Zone to pass over the Border Belt and thence 
into what, for want of a better name, we may for the present call the middle latitudes. 
In doing this, we shall have to consider and bring to light some features of the 
motions of the atmosphere in the middle latitudes more fully in detail than hitherto 
in the present paper. 
Let us accompany in thought the progress of a point advancing with the current 
along an average stream line, or rather an average current course in the great under¬ 
current from Polar to Equatorial Regions. For simplicity, let us confine attention to 
the Northern Hemisphere. Let us begin the course somewhere within the middle 
latitudes. To help imagination we may fix on a point of commencement situated 
vertically over New York. The moving point may, if we please, be idealized as being 
a small balloon constrained by frictionless guidance to keep in an average current 
course while being propelled along that average course by the more or less varying 
motions of the surrounding air. Now, during the progress of the travelling point 
in its course making way both eastward and southward, so long as the bottom lamina 
close to the surface of the Earth directly beneath the travelling point is blowing 
eastward with over-par revolutional velocity, the air above the bottom lamina there 
must be going forward with still greater over-par velocity. The reason for this state¬ 
ment is, that the only cause for maintenance of eastward relative motion in the 
frictionally restrained bottom lamina is, that the air above in virtue of revolutional 
momentum brought from equatorial regions, and not yet exhausted, is blowing with 
over-par revolutional velocity, and driving forward the resisted lamina below. Also, 
as long as the over-par velocity is existing in the frictionally resisted bottom lamina 
under our travelling point, a flow pole-ward also must exist in that bottom lamina at 
the place, for the time being, directly below the travelling point. This is for reasons 
fully explained in the account already given of my own theory. When further, the 
travelling point, in making its way southward, arrives at a stage where that eastward 
bottom over-par motion no longer exists directly below, and the travelling point then 
goes on making progress further south, it comes to places where the bottom lamina at 
the place then below it is moving equator-ward because of indraught thither, and 
because all reason for that lamina’s going northward has ceased. Thereafter in the 
* This equatorial belt of rising air may also well be called the Medial Belt, while the Calms of Cancer 
and Capricorn may be referred to as the Border Belts, this last especially when it is wished to speak of 
either of these two indifferently, without distinction as to whether it be in the northern or southern 
hemisphere. Also, either of these border belts may very well be described, or may be named when 
desirable, as the Belt of Offturn Parting or briefly as the Offturn Parting. The reason for this name will 
be seen readily by inspec tion of the diagram, where the great return current towards the Equator is 
parted into two currents, one going on southwards, and the other turning off towards the north. 
MDCCCXCII.—A. 4 S 
