575 
of Edinburgh, Session 1877 - 78 . 
depression, it is evident that the mode of inflow cannot be uniform 
all round, and that horizontal extension will take place in some 
one particular direction. This direction will be determined by the 
curve of outward propagation, which thus marks the direction of 
the onward course of the depression or storm. 
The Mode in which Opening Out takes place , accompanied by a 
transference of the Depression from West to East . — If a vessel be 
lowered into the sea a corresponding amount of water will be dis- 
placed. Let it then be supposed to move forward to the extent 
of its own length, an opening out ahead will take place, and a 
filling up astern. The original displacement will thus be closed 
up, but a similar displacement will be found in the new 
position to which the vessel has moved. Owing to its rigidity the 
vessel moves forward unaltered, but as the surrounding water is 
mobile, it cannot do so, hence it opens out and fills up. The currents 
which run aft on each side of the vessel represent its forward move- 
ment, while there is no real movement of the surrounding water, 
except its gradual transference to the rear, in the direction opposite 
to that in which the vessel moves. When an area of low pressure 
moves in an easterly direction over the British islands, and to a dis- 
tance equal to that of its own diameter, it will do so in a somewhat 
similar manner, viz., by opening out and filling up. There is, how- 
ever, an important difference to be noted ; for while the tranference 
of the water, which enables the vessel to move forward, takes place 
on each side in the direction of the stern, the transference of the 
air, when the depression moves eastwards, takes place on the north 
segment from the front, where it opens out to the rear where it fills 
up, as all observation shows. It may be pointed out here that 
while depressions move forward, the aerial particles, or the mass of 
air, does not move forward, and viewed in this light the curve of 
outward propagation may be regarded as being a purely ideal curve. 
The severe N.E. storms, which are of so frequent occurrence as 
to form a marked feature of the climate of the New England States, 
were long ago pointed out to advance on this region in an opposite 
direction from that in which they blow. Let the area of the storm be 
represented by the circle A B C E (fig. 4), then the front part ABC 
is the curve of outward propagation, which moves toward the N.E. in 
the direction of the arrow I) E, this being the contrary direction to 
