VOYAGE TO ALGERIA. 173 



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 considerable 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 colour as the plate sank, but at its 

 greatest depth, even in indigo water, the colour 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 pes- 

 ters 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 re- 

 move 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 colour 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 ultra- 

 marine. The surface of the water above the screw- 

 blade was always ruffled. Liquid lenses were thus 

 formed, by which the coloured light was withdrawn 



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

 green and blue. 



