126 The Colour of the Sea 



water. This proved to be a perfect observation tube for 

 determining the true colour of the water. Its diameter 

 was about 6 feet; it passed from the upper deck through 

 the captain's cabin on the main deck and the ward-room 

 on the lower deck into the water. Looked into from the 

 deck, the sea-water appeared to be enclosed in it as the water 

 is in a well, but with this difference, that the water, by day, 

 was brilliantly illuminated from below. There being no 

 clearance between the surface of the water in the well and 

 the structure of the ship, no light could enter except through 

 the water. No direct sky-light could reach it down the well, 

 because the poop awning, which was practically always spread 

 during the day, completely excluded it. The screw-well was, 

 in effect, an artificial and perfected Grotto di Capri, which 

 was carried round the world. It was perfected, inasmuch 

 as there is a passage for boats to penetrate into the grotto 

 from the outside, while the screw-well is entirely shut off. 

 During the whole of the voyage the colour of the water was 

 under observation in this very perfect apparatus. 



The statement that the blue colour of the sea is nothing but 

 the reflection of the blue of the sky was at first frequently made, 

 even on days when the sky was completely overcast; a visit 

 to the screw-well, especially on overcast days, never failed 

 to convince the doubter that the water contained in its own 

 mass sufficient colour to account for all that was perceived. 

 When the ship was in green water the view was never advanced 

 that its colour was due to reflection from the sky. 



As ships with screw-wells long ago disappeared from the 

 sea, it may not be superfluous to point out that what could 

 be observed in the screw-well was altogether different from 

 what can be seen in the wake of the screw of a modern steamer. 

 While the screw-well was a perfect instrument for gauging 

 the colour of the water, the determination of its transparency 

 was more conveniently made from a boat. Thus in mid- 

 Pacific, with the aid of a "water-glass" to eliminate the 

 disturbing action of ripples, a metal plate measuring only 

 4 by 4 inches, painted white and not masked by the suspending 

 line, was distinctly seen at a depth of 25 fathoms (45 metres). 



