281 
of Edinburgh , Session 1878-79. 
may be represented by the thickness of a sheet of paper spread over 
an ordinary globe. The difference in the mechanical effects which 
must then take place are evidently of great importance. In this 
latter case, the effect of the earth’s rotation will be fully intro- 
duced, and the inflow of the winds to the low barometric centre will 
here have to pass over a great extent of resisting surface, the effect 
of which was shown. Under these circumstances, a depression will 
not, as in the former case, tend to fill up ; it will now tend to open 
out, and in one particular direction, which is that of progressive 
movement. It does not move as a rigid aerial cavity forced onwards 
by high pressure in the rear, as it is by many supposed to do, but 
not being a rigid aerial cavity, it can only advance, as it sometimes 
does, at 70 miles an hour, as a circular atmospheric wave , by opening 
out in front, which may be regarded as being lateral and horizontal 
extension, and by filling up in the rear. It is difficult to conceive 
how progress can take place in any other way. This mode of advance 
was shown to take place only on a resisting surface, and not on a 
frictionless surface, on which depressions could not advance at all ; 
although the universally received opinion is, that a resisting surface 
retards progressive movement instead of facilitating it. Storms of 
the usual diameter move in direct contact with the surface of the 
earth, and not over a mass of calm air resting upon it. Query, Can 
they move through space 1 Conclusions were arrived at in a former 
paper on the barometer, that its rise and fall was greatly due to the 
passage of the air over a resisting surface ; on a frictionless surface 
this would be greatly diminished, and consequently much less 
difference would then be found to exist betwixt high and low- 
pressure, which causes and accompanies storms ; but if this is not 
to be found, it will then show one of the reasons why they do not 
move over a frictionless surface, on which they cannot be opened 
out by the rotation of the earth. 
The motive force which causes storms to move, is here supposed to 
be a central ascending current which carries off the spiral inflowing 
winds, which in the different segments are not uniform, either as 
to their direction or their velocity. Movement takes place in the 
direction in which supply is least copious, where the isobars are 
widest, and where inflow is most direct, as shown by Eev, W. 
Clement Ley in the Scottish Meteorological Magazine , w T ith, of 
2 K 
VOL. x. 
