FOG OF 1S31 HAEMATTAX. 



irregularly, and in such a manner as to "be quite incompatible with 

 the cometary hypothesis. 



The cometary hypothesis, then, being rejected, it has been sug- 

 d that these fogs may have had much nearer and less extra- 

 ordinary causes. It was recorded that great physical commotions 

 were manifested at opposite extremities of Europe in the year 

 1783. In the month of February terrible and long-continued 

 shocks of an earthquake took place in Calabria, which produced 

 great devastation, and by which more than 40,000 inhabitants 

 of that country were buried under the ruins of overturned 

 houses and buildings, and in the profound crevices of the cracked 

 crust of the earth. At a later part of the year, Mount Hecla 

 underwent the most violent eruptions ever witnessed, and new 

 craters were opened at various points at the bottom of the sur- 

 rounding sea, and even at considerable distances from the shore. 



Considering these and like commotions, it has been suggested 

 that the vapour, smoke, and gaseous matter ejected in enormous 

 quantities during such eruptions, dissipated by the winds, might 

 have been diffused through the atmosphere over the countries 

 where the fog prevailed. 



Another supposition assigns these fogs to the same cause as that 

 which produces showers of meteoric stones, noticed in another 

 number of this series. Among the various forms assumed by this 

 class of bodies, that of showers of fine dust is not unusual. Now, 

 we have only to admit the possibility of a still greater degree of 

 attenuation, to reduce such dust to the condition of the matter 

 composing a dry fog. This explanation would be quite compatible 

 with the local and unequal distribution of the phenomenon. 



Several medical authorities conjectured that the fog of 1831 

 might have been the cause of the epidemic cholera which prevailed 

 about that time. This supposition, however, is overturned by the 

 fact of the frequent prevalence of the same epidemic since then, at 

 epochs at which no such fogs were seen. 



12. Nevertheless, facts are recorded which render it certain 

 the atmospheric disturbances and currents do produce extraordi- 

 nary and hitherto unexplained effects upon epidemic diseases. A 

 very curious and remarkable instance of this influence is quoted 

 by M. Arago from the narrative of Matthew Dobson, an English 

 traveller. 



"A periodical wind, called Harmattan, blows three or four 

 times a-year from the interior of the African continent towards 

 the Atlantic coast, between latitudes 15 north, and 1 south. 

 The periods of its prevalence are stated to be chiefly from the end 

 of November to the beginning of April, its direction varying from 

 east-south-east to north-north-east. Its duration at any one time 



85 



