682 
PROFESSOR JAMES THOMSON ON THE 
Trade-Wind region now entered upon the surface of the Earth is dragging the bottom 
air forward revolutionally, and so is helping it briskly towards the Equator through 
increasing its centrifugal tendency. 
Then we have to notice that the air, during its course equatorwards and back 
again through the Trade-Wind Zone, receives forward revolutional momentum through 
the frictional forward drag applied to it by the Earth’s surface, and it loses no 
revolutional momentum, as the vacuum above the atmosphere can take none from 
it. So in departing northwards, as the grand upper current, it must carry with it 
far more revolutional momentum than it had in entering, as the great under current 
from the north across the Border Belt; but that great under current in entering was 
either at par, or partly at par, and partly at over-par, of revolutional velocity; 
consequently the grand upper current must depart across the Border Belt with great 
over-par of revolutional velocity. 
It follows from this, as a corollary, that the top of the atmosphere, or any 
isobaric interface near the top, must have a declivit}^ in approaching the Border 
Belt from the top of the Equatorial Belt; and the Border Belt must not have a 
maximum height with declivity thence to a minimum at the Equator. 
The foregoing demonstration seems also likely to give help towards the proper inter¬ 
pretation to be put on observational results recorded by the Krakatoa Committee, and 
to render highly improbable any suggestions such as seem to be conveyed in some 
parts of the report, to the effect that in the very lofty regions of the atmosphere—at 
such elevations as 13 miles above the sea level—a velocity such as 70 miles per hour 
from east to west has been indicated in the atmosphere, through the phenomena 
manifested after the great Krakatoa eruption. 
(c .) In connection with the reasoning or demonstration I have just given, there is 
another element which I regard as forming part of the whole truth, and which must, 
I think, form an important element towards the development of the theory more 
completely. I have already indicated in the demonstration just now offered, that the 
bottom lamina of the atmosphere in the trade-wind region, is especially helped to 
advance towards the Equator by the increased absolute centrifugal tendency super¬ 
imposed on it by the forward revolutional drag it there receives from the Earth’s 
surface; and which communicates to it, throughout its course towards the Equator, 
new accessions of revolutional momentum, and prevents it from getting into under- 
par of revolutional velocity, so much as does the air above it in the great under¬ 
current towards the Equator. For ready apprehension of this, it is well to notice 
that the under-par and increased under-par of revolutional velocity imply westward 
relative motion in the bottom lamina, and quicker westward, relative motion in the 
air next above within the great under current towards the Equator. 
This greater abatement of absolute revolutional velocity below par, or increase of 
relative velocity westward, constitutes a condition opposing flow towards the Equator 
in the main body of the great under current, and we may reasonably suppose that 
