FRESH-WATER ALG. 257 
navigators. Along with various species of ani- 
malcules, they are the cause of that peculiar 
olive-green tinge which extends over .a portion of 
the Arctic ocean, amounting to not less than 
20,000 square miles, every two miles of which, 
according to Scoresby’s estimate, comprehends 
23,888,000,000,000,000,—a number which would 
have employed 80,000 persons since the creation 
to reckon!! In the Antarctic Ocean, on the other 
hand, far beyond the limits where even the hardy 
lichen, moss, and sea-weed refuse to vegetate 
upon the rocks, and where every circumstance 
would seem inimical to the growth and propaga- 
tion of even the simplest plants, they occur in 
countless myriads on the floating ice, and cover 
the sea with meadows of a pale-brown hue, 
extending as far as the eye can reach, and down 
from the surface of the water to abysses deeper 
than plummet ever sounded. They form an 
1 Scoresby says, ‘After a long run through water of the common 
blue colour, the sea became green and less transparent. The 
colour was nearly grass-green with a shade of black. Sometimes 
the transition between the green and blue water is progressive, 
passing through the intermediate shades in the space of ten or 
twelve miles; at others it is so sudden that the line of separation is 
seen like the ripple of a current, and the two qualities of water 
keep apparently as distinct as the waters of a large muddy river on 
first entering the sea. In 1817, I fell in with such narrow stripes 
of various coloured water, that we passed streams of pale-green, 
olive-green, and transparent blue, in the course of ten minutes’ 
sailing.’ 
R 
