658 
PROFESSOR JAMES THOMSON ON THE 
diminished pressure at the base of the ascending heated current, and consequently 
entailing an influx at bottom from the lower part of the atmosphere at the colder 
places where descending currents are generated. In applying this general principle 
further to the explanation of the observed winds, he rightly explained the influx of 
the air from both sides towards the Equator or some medial part of the trade-wind 
region as being due to the more intense heating effects of the Sun in the Equatorial 
regions. But in the more important, because less obvious, element for explanation of 
the Trade Winds and of atmospheric circulation generally—that which is requisite for 
explaining the east to west motions of the Trade Winds, and the prevalence of winds 
from west to east in higher latitudes—he quite missed the true explanation. He 
attributed the east to west flow of the Trade Winds to the diurnal revolution round 
the equatorial zone from east to west of the maximum of accumulation of heating 
effect from the daily sunshine, which gives an accumulation of heat in the afternoon 
in each successive locality. Briefly, he said to the effect that as the maximum of 
accumulated heat runs round the Torrid Zone from east to west, passing each place at 
a few hours after noon of that place, and as the maximum of heat in travelling round 
always causes an indraught towards itself, so the atmosphere of the Torrid Zone must 
be brought into flowing round from east to west likewise. But this conclusion from 
the submitted premises is really quite inconsequential. 
In inference to this speculation, and treating for the present the direction which we 
will call the forward direction round the Torrid Zone as being that of the Sun’s progress 
from east to west, we may entertain considerations such as the following :—That 
consequent on the indraught from all sides towards the hot region, where the baro¬ 
metric pressure is most reduced, the backward-tending forces acting on the air in 
front of the maximum may be acting as much in respect to time and duration back¬ 
ward on the air in front of the maximum as do the forward-tending forces on the air 
behind that maximum, and that, through this consideration by itself, we might not 
be entitled to suppose that any resultant tendency to the generation of a current 
round the Torrid Zone one way or other, east to west, or west to east, would be pro¬ 
duced. But when we further consider the unsymmetrical character of the conditions 
of the two influxes towards the maximum region from before and from behind, and 
the to us very unknown accompanjdng frictional conditions between these unsym- 
metrically conditioned currents of air and the surface of the earth or sea over which 
they pass, we may be led to think it very unlikely that the forwarding and back- 
warding influences would exactly counteract one another ; and I certainly think they 
would not do so, and I think some resultant flow from east to west, or from west to 
east would be produced, but in which way, east to west or west to east, it would 
occur I am quite unprepared to say. # 
* As a matter of curiosity I think it might he interesting in a time of comparative leisure for some 
person to make experiments with a spirit lamp or other heater kept revolving slowly round in a circular 
path under a circular tray filled with water, the path being of a little smaller radius than the tray. The 
