On the Colour of the Atrnoftphere and Deep Water. 353 



of the sea was white, the waters near the margin would present 

 the same green tint which the white plate produced at different 

 depths ; but the bottom is usually of a dark grey which reflects 

 the light imperfectly, and can give rise only to dark and inde- 

 terminate shades of green ; it is therefore to the reflection of the 

 bottom that the green colour of the sea near the shores is to be 

 ascribed. In order to leave no doubt on this matter, and to 

 confirm an observation often made before, I took a boat and 

 proceeded from the shore, during a fine sunshine in July, at 

 eleven o'clock in the forenoon, with the view of examining at- 

 tentively the changes which should take place in the colour of the 

 water, while looking under the boat on the side opposite the sun. 



At about fifty toises from the shore, the water was of a de- 

 cided green, and this tint continued for a quarter of an hour; it 

 then became of a bluish-green ; as we advanced the blue gradu- 

 ally predominated ; and after sailing an hour the water under 

 the boat was of a fine blue, without the least mixture of green. 



While returning, I was careful to observe the reappearanci^ 

 of the green, and when I found the tint well defined, I ascer- 

 tained the depth, by sounding, to be 150 feet; the light of the sun 

 which produced the green colour- had thus to traverse 300 feet of 

 water. But in this part of the gulf, another cause contributes 

 to produce the green colour, viz. the impurity of the water for 

 several miles along the shore. This will not excite surprise 

 when it is considered that the sea of Naples receives no river that 

 can give rise to any motion in its waters, and that all the filth 

 of this populous town is thrown into it. On the shores of the 

 island of Capri the sea is perfectly blue at the depth of 80 feet, 

 because it is always pure ; while near Naples it was green at 

 150 feet — a difference which can only be attributed to the im- 

 purity of the water near the town at the time the. observation 

 was made. 



Water, however, may be blue at a much smaller depth than 

 80 feet, provided the bottom be black or of a very sombre hue, 

 so as not to reflect the transmitted light. 



If some obstacle intercept the direct rays of the sun, in such 

 a manner that the bottom is thrown into the shade, while the 

 water itself continues exposed to the light, the latter will be 



VOL. XV. KO. XXX. OCTOBER 1833. Z 



