VOYAGE TO ALGERIA. 131 
The green color of the sea has now to be accounted for; 
and here, again, let us fall back upon the sure basis of ex- 
periment. A strong white dinner-plate had a lead weight 
securely fastened to it. Fifty or sixty yards of strong 
hempen line were attached to the plate. My assistant, 
Thorogood, occupied a boat, fastened as usual to the davits 
of the Urgent, while I occupied a second boat nearer the 
stern of the ship. He cast the plate as a mariner heaves 
the lead, and by the time it reached me it had sunk a con- 
siderable depth in the water. In all cases the hue of this 
plate was green. Even when the sea was of the darkest 
indigo, the green was vivid and pronounced. I could 
notice the gradual deepening of the color as the plate sank, 
but at its greatest depth, even in indigo water, the color was 
still a blue-green.* 
Other observations confirmed this one. The Urgent is a 
screw steamer, and right over the blades of the screw was 
an orifice called the screw-well through which one could 
look from the pjoop down upon the screw. The surface- 
glimmer, which so pesters the eye, was here in a great 
measure removed. Midway down, a plank crossed the 
screw-well from side to side; on this I placed myself and 
observed the action of the screw underneath. The eye was 
rendered sensitive by the moderation of the light; and, to 
remove still furtherall disturbing causes, Lieutenant Walton 
had a sail and tarpaulin thrown over the mouth of the well. 
Underneath this I perched myself on the plank and 
watched the screw. In an indigo sea the play of color was 
indescribably beautiful, and the contrast between the 
water, which had the screw blades, and that which had the 
bottom of the ocean, as a background, was extraordinary. 
The one was of the most brilliant green, the other of the 
deepest ultramarine. The surface of the water above the 
screw-blade was always ruffled. Liquid lenses were thus 
formed, by which the colored light was withdrawn from 
some places and concentrated upon others, the water flash- 
ing with metallic luster. The screw-blades in this case 
played the part of the dinner-plate in the former case, and 
there were other instances of a similar kind. The white 
bellies of porpoises showed the green hue, varying in inten- 
* In no case, of course, is the green pure, but a mixture of green 
and blue. 
