Force of the Earth's Magnetism. 31 



observation would make the time of vibration of the general 



comparing needle 287".6 at Hammerfest. We have thus — 



In 1825, as above . . . 287". 6 



In 1827, M. Keilhau . . . 289". 6 



In 1823, Capt. Sabine . . 287". 1 



These results approximate perhaps as nearly as can be ex- 

 pected in the high latitudes, where the small, secular, and 

 (probably) diurnal variations in the dip, occasion much greater 

 alterations in the horizontal intensity than take place in the 

 lower latitudes. 



A remarkable Phenomenon of Sound, and of the conveyance of 

 minutely divided Matter, during the Eruption of Mount 

 Souffre in 1812. 



From many observations, I have found that thunder can rarely 

 be heard at a greater distance than twenty miles. I have also 

 had occasion to notice, that the eight o'clock gun of George- 

 town, in Demerara, is often heard at Cape Batave on the west 

 coast of Essequibo, a distance of about forty miles. The 

 distances, however, were almost incredible to which the con- 

 clusive motions arising from the tremendous explosions which 

 occurred during the eruption of Mount SoufFre were propa- 

 gated through the atmosphere. These eruptions were heard 

 at the distance of 600 or 700 miles, namely as far as Cayenne, 

 at Varinas, and, it is said, at Santa Fe. 



The whole of the coast, and most of the West India islands, 

 were alarmed by loud reports seemingly of great guns, which 

 were universally supposed to be caused by a sea engagement ; 

 and the ships of war among the islands and on the coast of 

 Guiana sailed out to reconnoitre, as they supposed, an enemy, 

 who, however, was nowhere to be found. 



These reports were very loud at Pomeroon, where I heard 

 them like the firing of cannon, incessantly for nearly two 

 hours. They excited so much consternation in the town, 

 from the idea of an engagement on land, as to put the troops 

 in motion. 



That sound should be conveyed, or, in other words, that a 



