578 Proceedings of the Royal Society 
uniformity in the mode of inflow, it would then tend to fill up. 
Progress is therefore the means by which a depression is maintained. 
Since the difference in the mode of inflow is due to a difference in 
the amount of friction called into play, we have here, probably, the 
reason why storms advance more rapidly over the land than over 
the ocean. The deflection of the winds produced by the earth’s 
rotation, and the irregularity in the amount of pressure at different 
points immediately outside the area of the depression itself, have 
doubtless important bearings on this whole question. 
It is very generally supposed that the spiral inflow to the centre 
is supposed to be there carried off by means of a vast ascending 
current from the centre. This opinion is, however, open to serious 
objections, but which it would be out of place here to discuss. 
The general result is, that progress is regarded as being due to an 
irregularity in the direction and force of the spirally inflowing currents, 
arising from the different degrees in which the element of friction 
is called into play in the different segments of the storm, accom- 
panied or aided by high pressure on the right of the direction in 
which the depression moves. 
Summary . — When a depression is formed with a narrow diameter, 
it fills up rapidly if it has a copious supply from the surrounding 
atmosphere, and the filling up is of course attended by a rapid 
rise of the low central barometer. But if the depression has a 
very large diameter, the element of friction must enter largely 
into the problem. The retarding influence due to this cause will 
necessarily be greatest in the denser currents near the earth’s 
surface ; while the light, rarefied, and mobile upper currents 
will move with great facility. Hence the main source of supply 
will be thrown on these freely flowing upper currents. The 
rapidity and facility with which they inflow give rise to horizontal 
extension or shallowing out aloft, and this extension would take 
place equally all round if the inflow were uniform in all the seg- 
ments of the depression. If, however, this uniformity of inflow 
does not exist, but is more copious in one segment and scarce in 
another, then extension and shallowing out will take place towards 
that particular direction where supply is scarce. Hence progressive 
movement takes place in this direction, and is attended by filling 
up in the rear. When supply is equally copious in all directions, 
