VOYAGE TO ALGERIA 185 



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 sur- 

 face-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, Lieu- 

 tenant 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 ruined. Liquid 

 lenses were thus formed, by which the colored light was 

 withdrawn from some places and concentrated upon others, 

 the water flashing with metallic lustre. 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 intensity as the creatures swung to and 

 fro between the surface and the deeper water. Foam, at 

 a certain depth below the surface, was also green. In a 

 rough sea the light which penetrated the summit of a wave 

 sometimes reached the eye, a beautiful green cap being 

 thus placed upon the wave, even in indigo water. 



But how is this color to be connected with the sus- 



