of Edinburgh, Session 1877 - 78 . 
573 
of the barometric minimum of a true tropical cyclone as in tig. 1. 
Let now DBF represent a vertical section of a barometric minimum 
of a very much larger diameter. When it opens up all round in the 
way described by means of rapid upper currents, it will be accompanied 
by a lowering of the surrounding gradients from the upper part 
of which the main source of supply is derived. The result is 
Fig. 2. 
that the depression originally embracing a circular space whose 
diameter was D F will widen out, and the diameter extend to G H. 
The inflow which in this way takes place along the gradients 
bounded by the circle whose diameter is D F, as shown by the 
arrows a and /, will now have the effect of lowering the gradients 
B F and B D to B H and B G. The result of this may be repre- 
sented by the removal of the air comprised in the spaces G B D 
and H B F. 
When the air moves more rapidly aloft than it does near the sur- 
face, it may be conceived as moving onwards, not in vertical, but 
in inclined columns ; and we have endeavoured to show* that this 
mode of inflow is attended by “lifting,” and to some extent by 
fictitious pressure, by which is meant that although the barometer 
indicates correctly the elasticity of the air, still it no longer 
represents its real mass overhead, but a pressure more or less 
diminished owing to the mechanical movement of the air and to 
friction. The opening out or extension of the area of barometric 
minimum, with D F for its diameter, to a wider area, having G H 
for its diameter, is effected by the upper currents, indicated by a and 
/ of fig. 2. These upper currents, which flow in upon the low 
central depression from a great distance all round, have their source 
of supply in the upper still atmosphere around and outside the cir- 
cular space over which they blow. It is here where outward exten- 
sion and shallowing out commences, viz., along the curve which 
4 G 
VOL. IX. 
* “ Proceedings,” vol. ix.p. 412. 
