666 
PROFESSOR JAMES THOMSON ON THE 
Also it was found that in the centre of a cyclone there is a region of comparative 
calm, and that the centre does not remain stationary, but travels at some moderate 
speed, taking generally a curved course over the surface of sea or land. 
The discovery also was established beyond room for doubt that cyclones in the 
northern hemisphere revolve in the direction opposite to that of the hands of a watch 
situated in their locality with its face up; while in the southern hemisphere they 
revolve in the same direction as do the hands of a watch situated in their locality 
with its face up. 
Also it was discovered and promulgated that in the central region of a cyclone the 
barometric pressure is remarkably diminished as compared with that of the general 
surrounding atmosphere, and that this condition must necessarily subsist as a con¬ 
comitant of the centrifugal tendency or “ centrifugal force ” of the revolving air, but 
whether the diminished pressure was to be regarded as a result of the centrifugal 
force of the revolving air, or as one of the primary causes of the institution of the 
cyclonic revolution, seems commonly to have been left unnoticed or to have been 
adverted to under erroneously imperfect views. 
Dove, for instance, when discussing the tremendously violent whirling motion 
which is met with in the inner part of a cyclone immediately around the central 
region of remarkable calm, says, “ the diminution of barometrical pressure is not the 
cause of the violent disturbance of the air, but rather a secondary effect of it,”* and 
through that passage with its context it seems doubtless that, while entertaining the 
view that the rapid revolving motion of the air somehow instituted maintains by 
centrifugal tendency the diminished pressure in the central region, he fails to notice 
the more complete truth, that without the actual occurrence of centripetal motion 
caused by predominating influence of inward suction the rapid revolving motion 
would not institute itself at all. 
This being said, however, there is yet, of the whole truth, another element which 
must be brought into notice, and which I here briefly describe, with some perhaps 
new ideas that have occurred to myself. 
It is, that while for a beginning an accumulation of buoyant air at bottom 
elongates itself upwards into a shape approaching to a columnar form, and so effects 
an abatement of pressure at its base ; and this abatement of pressure (or suction) 
induces a centripetal flow towards that place from outer regions where some slight, 
though it maybe almost imperceptible, motions having revolutional momentum (or, in 
other words, moment of momentum) round that place may already exist, and the 
revolving mass of air through the action of the centreward forces applied to it, takes 
an increasingly rapid revolving motion ; and further, this rapid motion reacts on the 
buoyant central column, keeping that from scattering through the air around it, and 
so institutes a very lofty continuous column of the buoyant air. 
* Dovk, ‘ Law of Storms,’English translation by Robert H. Scott, M.A., p. 198. 
