668 
PROFESSOR JAMES THOMSON ON THE 
higher or descend downwards, and may extend themselves greatly or contract in 
diameter, and which may be “parallel to the surface of the globe’' or “inclined 
forwards he goes on to say : “It appears to me that a simple flattened spiral stream 
of electric fluid generated above in a broad disk, and descending to the surface of the 
Earth, may amply, and simply, account for the commencement of a Cyclone.” # 
After making careful search through numerous writings on the subject of cyclones, 
1 have to say that I have no reason to think that the investigators who took 
part in the discovery of the directions of turning of cyclones in the northern and 
southern hemispheres had generally, or that any of them in particular had, any clear 
dynamic theory explanatory of the connection between these modes of turning and 
the rotation of the Earth, nor even of the origin of the very rapid whirling motion 
itself, but I have found strong indications of deficiency of such knowledge. Even 
Herschel, so late as 1857, in his article on “Meteorology,” in the ‘Encyclopaedia 
Britannica,’+ stated that a complete account of the phenomena of cyclones had been 
afforded “ by Hadley’s theory as developed by Dove in his ‘ Law of Dotation,’ 
and applied to this specific class of aerial movements by Professor Taylor,” and 
then went on to give what we may presume to be that explanation, but the ex¬ 
planation he gives, although containing enough of truth to prove the connection 
between the direction of the Earth’s rotation and that of the mode of turning 1 of 
cyclones in each hemisphere, is incomplete, and is vitiated by important errors of 
principle. 
Mr. Wm, Ferrel, of Nashville, Tennessee, in a paper of date 1856 (to be referred 
to further on in connection with other matters), adduced dynamic considerations of 
more advanced character for explanation of causes of the gyratory motions of cyclones ; 
but his treatment, although in some respects usefully suggestive and indicating 
sufficient reason for the direction of turning in each hemisphere, I cannot regard as 
being on the whole to very good effect. 
Also, as a further result of the researches and scrutinies and efforts towards 
generalization told of already, it came gradually into notice and into acceptance as an 
established truth that in the latitudes outside the limits of the Trade Winds extending 
far towards the poles, sometimes for brevity called the middle latitudes, the wind, 
while prevailing from the west as had been long previously known, prevails also for 
each hemisphere more from the Equator towards the Pole than from the Pole towards 
the Equator, so that, on the whole, to take for simplicity the case of the northern 
hemisphere, the prevalent average atmospheric current at the surface of the Earth in 
those latitudes was judged to be from the south-west; or, rather, without particu¬ 
larizing one exact point of the compass, and with allowance for great variations in 
different localities, and at different times, w^e may better say from south of west 
towards north of east. 
* ‘ Sailor’s Hornbook,’ third edition, p. 338, section 4-10. 
f 1 Encyc. Brit.,’ eighth edition, vol. 14, p. 650. 
