184 OBSERVATIONS ON THE WINDS. 



Mr. Ley remarks : — " The district in which the condensation is most 

 active occupies the front of the Cyclone. A little consideration will show 

 that over the North Temperate Zone it is on the Eastern side of a Cyclone 

 that this condition will in ordinary cases occur, since it is on this side that 

 we have Southerly winds, while these winds, in moving from warm to 

 colder regions, experience a diminution in their capacity for holding water 

 vapour. 



"A Cyclone must not therefore be regarded as a revolving disk, the 

 whole of which is propelled over the earth's surface, but rather as an eddy 

 of spirally ascending air, which eddy is being constantly reproduced on one 

 side of its original position. Fresh portions of the atmosphere are thus 

 perpetually thrown into cyclonic circulation, and it is only by a metaphor 

 that a progressive Cyclone can be said to preserve its identity during its 

 progress. 



" Cyclonic systems, exhibit the tendency to run in a series, a fact which 

 it is difficult to explain, if a Cyclone is regarded simply as a revolving disk. 

 When the atmosphere over an extensive district is much charged with 

 water vapour, the formation of a large nimbus is sufficient to originate a 

 Cyclonic system ; but a considerable portion of the atmosphere which is 

 drawn into the circulation never reaches the front of the Cyclone at all, 

 being left behind, as the Cyclone is rapidly developed over another part of 

 the earth's surface; this part of the atmosphere, therefore, falls calm before 

 it has parted with its aqueous vapour, and is in the condition favourable 

 for the propagation of another system. 



" An objection to the condensation theory of Cyclones is occasionally 

 drawn from the fact that in some cases both heavy and extensive rainfall 

 is found to occur in the immediate rear of a centre of depression, when 

 travelling Eastwards. Recent investigations, however, render it probable 

 that the axis of a progressive Cyclone is commonly so inclined, that at the 

 height of a few thousand feet above the earth's surface the area of least 

 pressure lags considerably behind the area of least pressure observed at 

 the earth's surface. It will therefore occasionally happen that, over the 

 wind which blows in the rear of a depression, currents flow in nearly an 

 opposite direction to that wind. These latter currents consequently pre- 

 cipitate their water through the lower stratum of relatively dry air." 



Bearing of the CcJitre. — In accordance with Buys-Ballot's Law, a person 

 standing with his back to the wind, in the Northern Hemisphere, near any 

 low barometric area, will have the centre about two points to the front of 

 his left hand, or, in other words, with a ship running before the wind, 

 about two points before the port beam. By the direction of the wind he 

 may know in what part of a Cyclonic area his vessel is ; by his barometer 

 he can tell whether the centre is approaching or leaving him; and knowing 

 the usual track of such Storms he can manoeuvre so as to avoid the centre. 

 (See the Section on Hurricanes or Tropical Cyclones hereafter). 



As the Cyclone moves along its course, it is evident that the barometer 

 will be falling more or less at every portion of the front, and rising more 

 or less everywhere in the rear, so that there must be a line of places some- 

 where across the Cyclone where the barometer has touched its lowest point 

 and is just going to rise. This line is called the " trough " of the Cyclone. 



