May 9, 1878] 



NATURE . 



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f .. As the centre of the main disturbance moves up 

 the great central valley the subsidiary centre east 

 of the AUeghanies moves with it, and where the 

 mountains decrease in elevation the two centres 

 draw towards each other so as to have a common 

 encircling isobar of 29"6o inches, and sometimes even 

 less. When they reach the latitude of New York, storms 

 of this type commonly leave the coast between latitudes 

 38° and 42°, attended by an area of high pressure imme- 

 diately to the northward, and followed by one from the 

 south-west. The courses of these storms across the 

 Atlantic are generally in comparatively low latitudes, 

 and they arrive on the British Coasts from the west or 

 south-west with moderate rains and winds backing from 

 the north-east to the north-west. 



Another type of Pacific storms is the one which 

 originates in the tropical zone of that ocean, and strikes 

 the Mexican coast, moving directly across that territory 

 into Southern Texas, and along the Gulf Coast over 

 Florida and Georgia to the Atlantic. The energy of 

 such storms is frequently very great, and they retain, 

 even after 'crossing the Mexican plateau, many of their 

 original cyclonic features. When they move north- 

 eastward through the Mississippi Valley they are always 

 attended by heavy rains and electrical disturbances. 

 Local storms or tornadoes are frequently developed on 

 their south-eastern margins during the spring and sum- 

 mer months, and are always very destructive. 



These Mexican storms, so called to distinguish them 

 from the disturbances that move over Northern Texas 

 from the California coast, will sometimes, but not often 

 cross the Alleghany Mountains from Tennessee to Vir- 

 ginia, and pass into the Atlantic northward of Cape 

 Hatteras. Their courses across the Atlantic are gene- 

 rally southerly as compared with those of storms leaving 

 Nova Scotia. They arrive on the British and French 

 coasts from the south-west, but are now and then carried 

 in a north-easterly direction, passing to the Norwegian 

 coasts northward of Scotland, and thence over the Scan- 

 dinavian Mountains into North-Eastern Russia and the 

 Siberian Seas, 



The cyclone, or great storm thatoriginates in the equa- 

 torial zone of the Atlantic, by which I mean the region 

 embraced between the equator and 15° N, lat., possesses 

 characteristics which mark it as the most destructive 

 atmospheric disturbance known to meteorologists. Of 

 course these storms are developed in the equatorial zones 

 of other oceans, but are not of such immediate interest 

 to us as the Atlantic cyclones. I am convinced that the 

 conditions which combine to develop nearly all areas of 

 low pressure have an equatorial origin, the exceptional 

 cases being due to local liberations of terrestrial heat 

 during earthquakes and to the heating of volumes of air 

 over great areas of sandy desert. North Atlantic cyclones 

 may be divided into four classes, namely : Those that 

 originate near the Cape Verde Islands and make their 

 northward curves east of the 35th meridian, and do not 

 affect the American coasts, but enter the European area 

 over Morocco and Spain, passing eastward over the 

 Mediterranean Sea. They are of comparatively rare 

 occurrence. Secondly, those that originate about the 

 40th, and curve northward east of the 8oth meridian. 



affecting the American coasts only by the induced mar- 

 ginal winds. Thirdly, those that originate immediately 

 east of the Caribbee or Windward Islands, and perform 

 their northward curves between the Both and 90th meri- 

 dians, so as to pass through the eastern part of the Gulf 

 of Mexico, and over Alabama, Florida, Georgia, and the 

 Carolinas toward the North Atlantic. Fourthly, those 

 that originate nearer to the equator than the others 

 referred to, and make the tremendous sweep from the 

 middle of the ocean between the Venezuelan coast of 

 South America and that of West Africa, over the West 

 Indian Islands to the Texas coast, and there curving 

 northward and eastward, sharply pass over the southern 

 sections of the United States and into the North Atlantic 

 from the vicinity of Cape Hatteras. 



Of the first-named class of cyclones, little need be said 

 beyond the reference already made. They represent the 

 most serious dangers to be encountered by vessels bound 

 to West African or South American ports, or passing 

 over the Cape route to the Indian Ocean. The second 

 class of cyclones, of which we have examples in the 

 great storms of October 12, 1780, August 17, 1827, and 

 August 12, 1837, and the later one as traced by the 

 United States Signal Service Bureau, which commenced 

 about August 18, 1873, take northerly courses. The only 

 land station where these can be accurately observed is 

 that at Bermuda ; therefore information regarding their 

 energy and movements must be collected from the logs 

 of ships that cross their tracks. It is believed that these 

 storms are developed only in the midsummer, and are 

 not of frequent occurrence, but on these points we have 

 very little reliable information. I am, however, inclined 

 to accept the statement as to their infrequency. 



The third class of cyclones we are more familiar with, 

 because it embraces that type of equatorial storm which 

 we most frequently experience. Examples from the 

 earlier meteorological records are the storms of August 

 10, 1831, and October 6, 1846. With these we have the 

 recent one of September 21, 1877, and which was signalled 

 to London by the Herald Weather Biureau. The passage 

 of this storm over the South Atlantic coast of the United 

 States was attended by many disasters, wrecks, and 

 inundations. Its course towards Europe was in compara- 

 tively low latitudes until it approached the Bay of Biscay, 

 when it moved sharply north-eastward, causing heavy 

 gales and rains, with thunder and lightning. The latter 

 effects were very marked in Scotland. 



The fourth class of cyclones, such as those of June 23, 

 1831, and September 27, 1837, and later on September 

 21, 1875, known as the great Galveston cyclone, are 

 usually of extraordinary violence. Among the first 

 successes of the Herald Weather Bureau was the correct 

 prediction of the course of this storm when it was moving 

 westward over the Carribean Sea. Only on one instance 

 within my observation has a cyclone of the third class 

 passed northward on the western side of the Alleghany 

 Mountains, and then the storm exhausted its energy in 

 Canada, but its depression, though much contracted, 

 reorganised into a minor disturbance when it passed into 

 the Atlantic, off the New England coast. The tendency 

 of cyclones to lose their force by the extension of their 

 area of low pressure is more decided than in any other 

 type of storm. This will account for the low degree of 



