634 DYNAMICAL GEOLOGY. 



8,9, Coscinodiscus atmospherica ; 10, Coscinodiscus (?) ; 11, Trachelomonas levis ; 12, 

 Campylodiscus clypeus ; 13-15, Gomphonema gracile; 16, 17, Cocconema cymbiforme; 

 18, Cymbella maculata ; 19, 20, Epithemia longicornis (frustule of E. Argus); 21, 22, 

 E. longicornis; 23, E. Argus; 24, E. longicwnis ; 25, Eunotia granulata (?) ; 26, E. 

 zebrina (?) ; 27, Himantidium Monodon (?) ; 28-32, Eunotia amphioxys ; 33, 34, Epiithemia 

 gibbenda; 35, Eunotia zebrina (?) ; 36, E. zygodon(?) ; 37, Epithemia gibba ; 38, Eu- 

 notia tridentula; 39, E. (?) levis; 40, Himantidium arcus ; 41, 42, Tabellaria; 43, 

 Odontidium (?) ; 44, Coccone'is lineata ; 45, C. atmospherica ; 46, Navicula bacillum ; 47,. 

 JV. amphioxys; 48, 49, jV. semen; 50, iV. serians; 51, Pinnularia borealis ; 52, P. 

 viridula; 53, P. viridis ; 54, Mastogloia (?) ; 55, Pinnularia mqualk(?) ; 56, Surirella 

 craticula (?) ; 57, 58, Synedra ulna; 59, Odontidium (?) ; 60, Fragilaria pinnata (?) ; 61 

 Mastogloia (?) ; 62-65, doubtful. 



A shower which happened near the Cape Verdes, and has been described by Darwin, 

 had by his estimate a breadth of more than 1,600 miles, — or, according to Tuckey, of 

 1,800 miles, — and reached 800 or 1,000 miles from the coast of Africa. These numbers 

 give an area of more than a million of square miles. 



Dust from a shower over Italy, in 1803, afforded Ehrenberg forty-nine species of 

 organisms, and another, in 1813, over Calabria, sixty-four species; and the two had 

 twenty-eight species in common. 



In 1755, there was a "blood-rain " near Lago Maggiore, in northern Italy, covering 

 about two hundred square leagues ; and at the same time nine feet of reddish snow felL 

 on the Alps. The earth-deposit in some places was an inch deep. Supposing it to 

 average but two lines in depth, it would be for each square mile an amount equal to 

 •2,700 cubic feet. The red color of the " blood-rain " is owing to the presence of some 

 red oxyd of iron. 



Ehrenberg enumerates a very large number of these showers, referring to Homer's 

 Iliad for one of the earliest known. With such facts before us, how many millions 

 of hundred-weight of microscopic organisms have reached Europe since the period of 

 Homer? The whole number of species made out is over three hundred. 



The species, so far as ascertained, are not African; fifteen are South American. But 

 the origin of the dust is yet unknown. The zone in which these showers occur covers 

 southern Europe and northern Africa, with the adjoining portion of the Atlantic, and 

 the corresponding latitudes in western and middle Asia. 



6. Changes of Atmospheric Pressure. — A local change of atmospheric 

 pressure, from a passing storm, has an eifect on any large body of 

 water beneath it, a diminution of pressure causing the water directly 

 beneath to rise from the greater pressure elsewhere. A variation of 

 one inch in the mercury column of a barometer is equivalent to 13.4 

 inches in a column of water. Captain J. C. Ross has observed, in the 

 Arctic regions, that a change of pressure of this kind was perceptible 

 in the tides. Observations through forty-seven days gave a variation 

 in the water of nine inches, corresponding to two thirds of an inch in 

 the barometer. 



The wind during storms produces sometimes an elevation of the 

 water in the leeward part of a lake, at the expense of that in the 

 other, as has often been observed in the Great Lakes of North Amer- 

 ica. Great waves on the ocean and extraordinary tides on sea-coasts 

 are other effects of the same cause. The subject of waves is treated 

 of under the head of Water. 



7. Chemical action. — The atmosphere also plays an important part 



