1820.] Royal Academy of Sciences. 383 



south of the equator, it is winter in the northern hemisphere. 

 The air of the temperate zone is then as different as it can be 

 from that of the torrid zone. There flows into the latter a con- 

 ^etant, cool, and uniform breeze, which carries the heated and 

 moist air into the higher regions, from whence it flows back 

 towards the same temperate zone, reestablishes the equilibrium, 

 and deposits its moisture there ; so that the mean heat is always 

 5° or 6 less in the dry season than in the rainy ; but the south- 

 east winds do not act like those of the north ; because they come 

 from a hemisphere which contains much more water, and in 

 which the upper current of air is not dispersed in the same 

 manner as in the northern hemisphere. 



M. Moreau de Jonnes has communicated some details, ex- 

 tracted from his correspondence, relative to the hurricane that 

 caused so much damage in the Caribbee islands the 21st of last 

 September. It was preceded by a dead calm, the wind shifted 

 from north to north-west, and it was in this point that it blew 

 with violence. M. de Jonnes remarks upon this subject, that in 

 the preceding year the hurricane of the 20th of October came 

 from the south west ; and that there exists a space of 90° 

 between these two points, from south to north, from whence the 

 wind never blows. The agitation of the air was followed by a 

 great swell of the sea, which caused the shipping to drive ; yet 

 no extraordinary movement was observed in the barometer. It 

 is remarked, with some degree of sorrow, that the effect usually 

 attributed to hurricanes, of purifying the air of the countries they 

 devastate, was not verified upon this occasion, for the yellow 

 fever did not cease to commit its usual ravages. 



The same observer has also given some notices respecting the 

 earthquakes which have been felt this year in (he Caribbee 

 Islands ; and which have had this peculiarity, that they have 

 affected a kind of periodical recurrence. Eight of these earth- 

 quakes were felt from December to May ; one every month, 

 except in April, in which month there were two ; and all of them 

 took place in the night between nine o'clock and eleven. 



MINERALOGY AND GEOLOGY. 



M. Beudant continues to enrich crystallography with re- 

 searches equally new and interesting. We saw last year, by his 

 experiments, how a saline principle of a certain kind sometimes 

 impressed its crystalline form upon a mixture in which it did not 

 by any means form the greatest part. 



He has occupied himself this year with a question that is not 

 less important in respect to the knowledge of crystals ; namely, 

 to determine the causes which occasion a saline substance, whose 

 primitive molecules and nucleus have a constant form, to be dis- 

 guised, by means of the accumulation of its molecules according 

 to different laws, with so many and so various secondary forms 

 that their number is sometimes astonishing. 



