THE ATMOSPHERE AS A MECHANICAL AGENT. 165 



48,49, .ZV. semen; 50, N. serians ; 51, Pinnularia borealis ; 52, P. viridula; 53, P. 

 viridis ; 54, Mastogloia (?); 55, Pinnularia cequalis (f); 56, Surirella craticula (?); 

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

 (?); 62-65, doubtful. 



A shower which happened near the Cape Verd Islands, and is described by Darwin, 

 had by his estimate a breadth of more than 1600 miles, — or, according to Tuckey, of 

 1800 miles, — and reached 800 or 1000 miles from the coast of Africa. These numbers 

 give an area of more than a million square miles. 



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

 about 200 square leagues, which made an earth deposit in some places an inch deep ; if 

 averaging two lines in depth, the amount for each square mile would equal 2700 cubic 

 feet. The red color of the "blood-rain" is owing to the presence of some red oxide of 

 iron. 



Ehrenberg enumerates a large number of these showers, citing one of the earliest 

 from Homer's Iliad ; and among those whose deposits he examined he distinguished over 

 300 species of organisms. The species, so far as ascertained by him, are not African, and 

 15 are South American. 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. 



Angle of Rest of Falling Sand or Gravel. 



The angle of rest in falling sand or gravel varies w^ith the size, density,, 

 shape, and smoothness of the grains ; and also with the amount of moisture 

 or water present among them, little moisture causing adhesion of grains, 

 much water producing a flowing mud. With no friction, as is essentially the 

 fact in the case of the particles of a liquid, like water, the angle is null ; 

 with ordinary dry sand, 30° to 35° ; with ordinary volcanic cinders, 33° to 40°. 



Instructive experiments may be made by inserting vertically a graduated rod at the 

 center of a circular board graduated similarly from its center outward, and then dropping 

 over the board about the rod sand of different kinds, the ratio of height to radius giving 

 the angle. The author obtained in this way for dry angular quartz sand about 0-005 inch 

 in radius, the angle 35° 20' to 36° 30' ; for iron- sand, of like fineness, 33° 10' to 33° 40' ; 

 for new (untarnished) shot. No. 10, very fine, 20° 12' ; same, No. 4 (coarser), 27° 50' ; 

 same. No. 3 (buck-shot), 29° 40' ; and with tarnished shot, a higher angle. 



When deposition is around a center, or pericentric, the resulting form is 

 approximately conical, varying with irregularities in deposition through the 

 winds and other causes. 



Atmospheric Pressure. 



Variations in atmospheric pressure, which may amount to two inches 

 of the barometer in a few hours, or half as many pounds per square inch, 

 are supposed to influence the resistance of the earth's crust to earthquake 

 movements, and to volcanic eruptions. The tide-like movements in large 

 lakes are attributed to other causes. 



