586 EEPOBT— 1888. 



1. 



MONDAY, SEPTEMBER 10. 



The following Reports and Papers were read : — 



Third Report of the Committee for inviting Designs for a good Differential 

 Gravity Meter. — See Reports, p. 72. 



Fourth Beport of the Committee for considering the lest means of Com- 

 paring and Reducing Magnetic Observations. — See Reports, p. 28. 



3. Third Report of the Committee appointed to co-operate tvith the Scottish 

 Meteorological Society in making Meteorological Observations on Ben 

 Nevis. — See Reports, p. 49. 



4. Modern Views about Hurricanes, as compared ivith the Older TJieories. 



By Hon. Ralph Abercrojibt, F.R.Met.Soc. 



The old conception of a hurricane was that of a circular-shaped eddy, round 

 which the wind blew in circles; the whole system was not supposed to be con- 

 nected with any surrounding trade-wind or monsoon, and the idea that a hurricane 

 changes it shape, as well as its depth and intensity during its progress, was never 

 thought of. 



Modern research shows that a hurricane is really an oval eddy, and that the 

 vortex, or centre of the wind rotation, is not in the geometrical centre of the oval, 

 but usually nearer one edge or other of the depression. The former is a very simple, 

 the latter a very difficult conception ; nevertheless, such are the facts, as the author 

 has proved by an examination of hurricanes on 60 different days. 



The wind blows as a spiral of variable incurvature round the vortex, not round 

 the centre of the oval. The general sense of the rotation is counter-clockwise in 

 theNorthern, clockwise in the Southern Hemisphere ; but the amount of incurvature 

 varies in different parts of the oval for a number of reasons. As a ride, in all hurri- 

 canes, the incurvature is less in front than in rear of the vortex. 



A hurricane is also always changing its shape, so that the oval lies sometimes 

 in one way, and sometimes quite in a different direction ; while sometimes the 

 vortex is displaced towards one side of the oval one day and towards quite another 

 side on the next. 



The path of the hurricane is not in a regular line, for the vortex sways about, 

 and sometimes even describes a loop. 



For all these reasons, no rule is possible for determining absolutely the bearing 

 of the vortex by observations on board a single ship ; whereas it used to be stated 

 positively that facing the wind the vortex bore 8 points — at right angles — to the 

 right in the Northern, and to the left in the Southern Hemisphere. 



AVe can only say now that, when fairly within the storm-field and facing the 

 wind, the vortex will be to the right and a" little to the rear ; that is, from 8 to 12 

 points to the right of the wind in the Northern Hemisphere ; and to the left and a 

 little to the rear ; that is, from 8 to 12 points to the left of the wind in the Southern 

 Hemisphere. If the wind blew exactly in a circle round a circular hurricane the 

 vortex would always bear 8 pobts to the right or left according to the Hemisphere, 

 and the rule to take 8 to 12 points is simply allowing for the effects of variable 

 incurvature. 



The above rule does very well for a first approximation to the bearing of the 

 vortex, but greater precision can be attained in certain circumstances. If the 

 condition indicate that a ship is nearly in front of the vortex, the bearing of tha 

 vortex will probably not be much more than 8 points to the right or left, according 

 to the Hemisphere, because, as before mentioned, the incui-vature is very small in 



