614 Proceedings of the Royal Society 
inodes of inflow towards the low central barometer; one is an 
advantageous, the other is a disadvantageous form. It is by this 
latter mode that the gradient is lowered. It takes place with 
inclined columns, resulting from rapid uppers and retarded surface 
currents. Much of the work of inflow is thus thrown upon the 
uppers. To enable them to maintain their superior velocity, they 
themselves must be adequately supplied by the uppers in advance. 
This is accomplished by outward extension ; they advance forwards 
to procure the requisite supply from the still atmosphere a-head, 
which now begins to inflow spirally. It is to this advancing 
line of removal, that the term “ Curve of Outward Propagation” 
is applied. It may be illustrated thus : — If a river flowing down 
an incline does so uniformly, and at am- equal rate of speed, 
removal will equal restoration; but if in the lower part of its 
course, a more rapid removal is inaugurated, while restoration or 
supply above remains as before, the curve representing the poin 
at which the increased removal begins to travel upwards will repre- 
sent the forward movement of this curve of outward propagation or 
extension. 
When a rapid fall of the barometer takes place, if the inflow to 
the depression so formed assumes an advantageous form, it will fill 
up at once ; but if, as usually takes place, it assumes the disad- 
vantageous form or mode of inflow, instead of filling up, it will 
open out and extend itself outwards all round, like the undulations 
produced by a stone thrown into still water. The uniformity of 
this extension will depend on the uniformity of the motive central 
inflow. With inflowing winds of different degrees of density, 
temperature, and moisture, it may be safely asserted that such 
uniformity of inflow will not occur. A disadvantageous mode of 
inflow will consequently take place in one segment, with a less 
disadvantageous mode in another. The first is found in the 
advancing segment, the latter in the rear. 
The effect of this want of uniformity in the mode of inflow will 
be, that the depression, instead of extending itself uniformly all 
round, will shallow itself out in one particular direction , which is that 
in which the disadvantageous mode of inflow is found, and where 
the curve of outward propagation exists. This disadvantageous 
mode of inflow is increased by the circumstance, that as the 
