577 
of Edinburgh, Session 1877 - 78 . 
progress of depressions over the ocean is 19 miles an hour, over 
the land the rate is 26 miles an hour. It is also evident that 
if a depression moves forward accompanied by spirally inflowing 
winds, — considered simply as inflowing air-currents, and not as 
representing an efficient motive force capable of generating forward 
movement, — a derangement as regards their force and direction 
would take place, especially on those occasions when the onward 
march of the storm attains a speed of 70 miles an hour. But in no 
case has such a derangement been observed. 
Proposed Theory as to the Progress of Storms. — When, however, 
the depression is not being impelled forward in the mode above 
described, but in virtue of an opening out in front, and when 
the air-currents, inflowing spirally upon the centre, are regarded 
as an effective motive force capable of causing progressive motion, 
no derangement can be supposed to take place as regards the 
force and direction of the air-currents ; and no steepening of the 
gradients in front will take place, since the change in the position 
of the depression is effected with facility by the transference of the 
air from the front to the rear along the north segment of the depres- 
sion. 
A depression advances by means of these spirally inflowing currents, 
owing to the difference which exists in their mode of inflow, and conse- 
quently in their force and direction ; and it is in this way that outward 
extension in one direction is carried out. This difference in the mode 
of inflow is mainly due to the introduction in a much higher degree 
of the element of friction into the currents of the front segment ; in 
this way scarcity of supply is produced in this part of the depres- 
sion, and the effect is intensified by the ascending tendency of the 
air in this segment of the depression. On the other hand, the 
currents which enter in at the rear are much less retarded by fric- 
tion, because there is a descending tendency of the air in this 
segment, and, consequently, supply is copious.* The low central 
pressure, by travelling onwards towards the region in front, where 
supply is scarce, and where opening out takes place, practically 
overcomes this scarcity of supply, and thus uniformity of inflow is 
practically restored by progressive movement. If there was no 
progress, and if the depression was accompanied by this want of 
* “Proceedings,” vol. viii. p. 613-615. 
