Tropical and Extra-tropical Cyclones. 5 



This line may be called the " trough " of a cyclone ; and the cha- 

 racteristic squall, the sudden rise of the barometer, and the jump of 

 the wind associated with the passage of that line, together with the 

 different character of the weather and clouds in front and rear of 

 that line, may conveniently be classed together as the " trough phe- 

 nomena" of a cyclone. 



The word " trough " requires some explanation, as that term has 

 been strongly objected to by some. The difficulty of realising the 

 applicability of the word arises from the difficulty of realising the 

 forward motion of a cyclone when looking only at a set of oval 

 isobars. But when it is considered that a cyclonic vortex is propa- 

 gated somewhat after the manner of a wave ; and that a barograph 

 during the passage of the cyclone traces out a wave-like curve, the 

 word trough comes in naturally to denote the line drawn across the 

 isobaric plan of a cyclone to show all the points where the minimum 

 has been attained, owing to the motion of the depression. 



A cyclone is really a complex moving eddy of air. For some 

 reason or other, pressure decreases under this eddy in a manner which 

 is conveniently mapped out by isobars. In ordinary language the 

 •oval isobars are called for shortness a cyclone, but of course they 

 are really only the symbol of what is taking place in the air over- 

 head. 



Isobars give the plan, barograms the sections of a cyclone, and it is 

 .always difficult to realise the relation of one to the other ; but there 

 is no difficulty in the conception of a moving vortex of air, which of 

 •course would have a trough like an ordinary wave. 



A stationary vortex, like a stationary cyclone, would have no 

 trough. Suppose a stationary cyclone to form over an observer and 

 then to die out. His barometer would first fall and then rise, 

 but there would be no trough, and he would experience no trough 

 phenomena. 



It is important to notice that the line of the trough does not appear 

 to be always at right angles to the path of a cyclone, whose longer 

 diameter is not perpendicular to the line of propagation ; and that 

 the trough phenomena are far more marked on the southern than on 

 the northern side of the centre in Great Britain. 



Taking a general view of the phenomena of a cyclone, they appear 

 to have a double symmetry — a symmetry round a point, and a symmetry 

 .about a line. The rain area, cloud ring, and general rotation of the wind 

 are obviously related to a point, the centre of the cyclone. On the 

 •contrary, the quality of the heat, the relative humidity, the character 

 of the clouds, and a particular line of squalls have nothing to do with 

 a point, but are related to the front and rear of the line of the trough. 



No attempt has been made as yet to discover the origin of these 

 trough phenomena ; but from the results of his researches in th 



