564 LECTURE LVII. 



has incroached in particular parts, and retired from others ; and the 

 mouths of large rivers, running through low countries, have often been 

 variously modified, by a deposition and transfer of the matter washed 

 down from the land. At Havre the sea undermines the steep coast, and 

 recedes at Dunkirk, where the shore is flat : in Holland the Zuyder Zee 

 was probably formed in the middle ages by continual irruptions of the 

 sea, where only the small lake Flevo had before existed ; and the mouths 

 of the Rhine have been considerably altered, both in their dimensions and 

 in their directions. The mud, deposited by large rivers, generally causes a 

 Delta, or triangular piece of land, to grow out into the sea ; thus the 

 mouth of the Mississippi is said to have advanced above 50 miles since the 

 discovery of America ; and the sea has retired from Rosetta above a mile 

 in 40 years. The mouths of the Arno and of the Rhone consist also in 

 great measure of new land.* 



The meteors denominated shooting stars are observed to move in all 

 directions, as well upwards as downwards, although they frequently seem 

 to have a tendency towards a particular quarter in the course of the same 

 evening. Their height is seldom less than 20 miles, and sometimes as 

 much as 100 or 200, but usually about 50 ; their velocity is commonly 

 about 20 miles in a second, which differs very little from that of the earth 

 in its orbit. The rapidity of their motion, as well as its occasional devia- 

 tion from a right line, has generally been considered as a reason for sup- 

 posing that they depend on electricity ; but the opinion is by no means 

 fully established. 



Other igneous meteors, which nearly resemble in their appearance the 

 largest of these, are sometimes observed to fall on the earth, either entire 

 or divided ; and after their fall, certain stones have been found, which have 

 been supposed to have descended in an ignited state.t Mr. Howard J has 

 ascertained that almost all these stones agree in their general characters, 

 and in their chemical analysis, especially in the circumstance of containing 

 nickel. It has been conjectured, both in this country and on the con- 

 tinent, that they have been emitted by lunar volcanos, and it has been 

 observed, that since they would find little or no resistance from the very 

 rare atmosphere of the moon, they would require a velocity of projection 

 only four times as great as that which a cannon ball sometimes receives, in 

 order to rise into the sphere of the earth's attraction. Their heat and 

 combustion may not improbably be derived from the great condensation 

 which they must occasion in the air immediately before them, and even 

 their friction might easily produce enough of electric light, to render them 

 visible in the dark. Among many such substances projected from the 

 moon, it is probable that a few only would be directed towards the earth, 

 and many more would be made to revolve in ellipses round it, and become 

 little satellites, too small for human observation, except when they enter 



* The reader is referred to Lyell's or Ansted's Geology. 



t On meteoric stones, see Chladni on the Siberian iron, Riga, 1794 ; Ueber 

 Feuer-Meteore, Vienna, 1819, with App. by Schreibers ; and art. Stones (Meteor.)* 

 Encyc. Metrop. 



J Ph. Tr. 1802, p. 168. 



