of the Atmosphere. 429 
scale the principal features of cyclonic storms and hurricanes. 
These-are always preceded by inequalities of temperature in the 
regions where they occur ; and it is extremely probable that such 
inequalities take place in a vertical as well as in a horizontal 
direction. The distribution of watery vapour must at the same 
time be affected ; and this would again react upon the equilibrium 
of the atmosphere, so as to favour the existence of ascending and 
descending currents. The rapid oscillations of the barometric 
column which usually precede hurricanes are thus doubtlessly 
connected, not only with variations in the statical pressure, but 
also with the irregular influence of vertical and oblique currents, 
which at such times disturb the equilibrium of the atmospherical 
column over the barometer. 
8. The duration and energy of many of the vertical currents 
which came under my observation, were such as to show that 
currents of a greater order than those which take place by the 
influence of the heated ground immediately beneath are some- 
times developed among the overlying atmospheric masses. Such 
currents being of much greater magnitude than those which would 
account for the rapid fluctuations of the thermometer already 
noticed, we may refer to them not only great interchanges of 
temperature in different strata of the atmosphere, but also a very 
efficient part in the production of ordinary winds. If an exten- 
sive portion of the earth’s surface becomes more heated than 
other surrounding portions, the air will ascend and overflow 
above the cooler air resting upon the unheated surfaces. The 
cold air at bottom will at the same time tend to rush inwards, so 
as to fill up the vacuum which the ascending currents would 
have left above the surface of the heated ground. As the air 
that overflows above does not rush into a vacuum, but penetrates 
and mingles with masses of cooler air possessing nearly the same 
density, its progress is considerably retarded, while at the same 
time some of the vapour which it may contain is condensed so 
as to assume a vesicular cloudy form. A corresponding retar- 
dation in the motions of the air rushing in from the colder to the 
warmer surface below, is also produced from the resistance of the 
air lying oyer the latter. The production of sea- and land-breezes 
furnishes a complete and instructive illustration of these remarks. 
Many of the upward currents which I observed with the anemo- 
scope during the summer mornings, were undoubtedly the pre- 
cursors of the sea-breeze. Such currents continue to accompany 
the production of the land- and sea-winds in a manner that I have 
sometimes been able to observe by the smoke of steam-vessels 
near the coast. Thus, on a warm day in June 1857, I observed 
the simultaneous existence of the sea-breeze at Kingstown and a 
slight motion of a few light clouds from the interior towards the 
