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 poop 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 further all 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 formed, by which the colored light was withdrawn from 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- 



* ID no case, of course, is the green pure, but a mixture of green 

 and blue. 



