18 PHYSICAL GEOGRAPHY OF THE SEA. 
sea-water, must be set vertically on a white plate, and then, 
looking through the open end, you will see the white of the 
porcelain changed into a light blue tint. 
In the Gulf of Naples, we find the inherent colour of the 
water exhibited to us by Nature on a most magnificent scale. 
The splendid “ Azure cave,” at Capri, might almost be said to 
have been created for the purpose. For many centuries its 
beauties had been veiled from man, as the narrow entrance is 
only a few feet above the level of the sea, and it was only 
discovered in the year 1826, by two Prussian artists accidentally 
swimming in the neighbourhood. Having passed the portal, 
the cave widens to grand proportions, 125 feet long, and 145 
feet broad, and except a small landing place on a projecting rock 
at the farther end, its precipitous walls are on all sides bathed 
by the influx of the waters, which in that sea are most remarkably 
clear, so that the smallest objects may be distinctly seen on the 
light bottom at a depth of several hundred feet. All the light 
that enters the grotto must penetrate the whole depth of the 
waters, probably several hundred feet, before it can be re- 
flected into the cave from the clear bottom, and it thus 
acquires so deep a tinge from the vast body of water through 
which it has passed, that the dark walls of the cavern are 
illumined by a radiance of the purest azure, and the. most 
differently coloured objects below the surface of the water are 
made to appear bright blue. Had Byron known of the exist- 
ence of this magic cave, Childe Harold would surely have sung 
its beauties in some of his most brilliant stanzas. 
All profound and clear seas are more or less of a deep blue 
colour, while, according to seamen, a green colour indicates 
soundings. The bright blue of the Mediterranean, so often 
vaunted by poets, is found all over the deep pure ocean, not 
only in the tropical and temperate zones, but also in the regions 
of eternal frost. Scoresby speaks with enthusiasm of the splendid 
blue of the Greenland seas, and all along the great ice-barrier 
which under 77° S, lat. obstructed the progress of Sir James 
Ross towards the pole, that illustrious navigator found the waters 
of as deep a blue as in the classical Mediterranean. The North 
Sea is green, partly from its water not being so clear, and partly 
from the reflection of its sandy bottom mixing with the essen- 
tially blue tint of the water. In the Bay of Loanga the sea has 
