282 
Proceedings of the Royal Society 
coarse, as above stated, exceptional cases, which may be afterwards 
explained. This is shown in the diagram of the ascending balloon. 
The rise and fall of the thermometer is equal both aboye and 
below its mean, but the extent of the rise and fall of the barometer is 
much greater below than above its mean. This was accounted for so 
far by “ lifting ” or fictitious pressure, which accompanies southerly 
winds, which prevail when the barometer is below its mean. 
The second part of this paper is intended to show why storms 
require high pressure to the right of the direction in which they 
advance. This is represented by a diagram of a flat wheel, which is 
laid horizontally on the surface of the ground ; it circulates round 
its centre, which also moves forward in one direction. It is in this 
way that a depression also moves, say, in an easterly direction. Its 
winds have a circular rotatory movement round its low centre in a 
direction opposite to that of the hands of a watch. Let their 
circular rate of speed be at 30 miles per hour, and let the forward 
movement of the low centre also take place at the same rate of 
speed. The consequence of this will be that east winds on the 
north segment will be at rest — calm — and not move over the 
ground ; at the same time they must necessarily form a segment of • 
the circular rotation round the low centre. If thus at rest, they 
must cause a collapse, and a termination of the circular rotation of 
the depression. But they do not do so. To account for this, these 
east winds may be regarded as being space winds, and as such in 
this way their circular rotation may be maintained. As space winds 
they actually move, though not over the surface by which their 
speed is estimated, and hence they may be said to blow when they 
do not blow. West winds on the south segment, having their 
velocity regarded as being made comparable with the mode in 
which east winds blow, when they are supposed to be at rest, may 
be said to blow more rapidly than they really do bloiv, because their 
speed is not merely to be represented by their circular rotation 
round the low centre, but also by the additional speed which they 
acquire in the direction of the advancing low centre. Under these 
circumstances, the velocity of east and ivest winds are not comparable 
as estimated by their passage over the surface. To render them 
comparatively equal, they must be regarded as being both space 
winds and earth winds. 
