571 
of Edinburgh, Session 1877-78. 
the other, of which our European storms are examples, and which 
alone are dealt with in this paper, is of very great extent as com- 
pared with the height. The modes in which these two distinct 
kinds of barometric depressions tend to fill up are widely different 
from each other, owing particularly to the degree in which the 
element of friction is introduced by the forces set in motion within 
the storm itself. 
Mode in which a Barometrical Depression of a Narrow Diameter 
fills up. — In this case, since the diameter of the depression is small 
as compared with the vertical height of the storm, it follows that 
the inflow of the air-currents towards the area of lower pressure 
will take place over a surface of comparatively small extent, and will 
consequently meet with but little retardation from friction. Hence 
the process of inflow toward the centre of the barometrical depression 
will take place with comparative facility in true tropical cyclones of 
small extent, and be therefore characterised by steep barometrical 
gradients. 
A C 
Fig. 1. 
Let ABC (fig. 1) represent a vertical section of such a depression, 
and let the small arrows a , b, c, show the direction in which it will 
tend to fill up over a surface of comparatively small extent, 
accompanied by a steepening of the gradients B A and B C. 
Mode in which a Barometrical Depression of Wide Diameter fills 
up and in the process tends to open out. — In European and American 
storms, the barometer at their centres falls about as low as in tropi- 
cal cyclones, but the barometric gradients are much less steep, and 
the disturbances cover a much more extensive region. Bedfield has 
pointed out that their vertical height is often not more than the two- 
hundredth part of their horizontal breadth. It follows from this 
essential difference between these storms and true cyclones that 
the mode of inflow of the air-currents towards the central low 
pressure can neither take place in the same manner nor with a 
