1867.] On the Luminosity of the Sea. 503 



striking, appears to be completely extinguished by moonlight, even 

 when the moon is young. It appeared, only less marked, on the 

 two following evenings, and on the third we left the bay. I am 

 informed that Simon's Bay has been remarked as frequently exhibit- 

 ing this phenomenon. 



On the 7th of July, in lat. 28° N., on the coast of China, two 

 days after the occurrence of this form of luminosity, as before 

 noticed, a heavy swell coming in from the S.W. was met by a N.E. 

 wind, and the ship rolled tremendously. The sea was beautifully 

 luminous, every wave breaking into a pale light which was visible 

 at a considerable distance, so that the whole sea was streaked with 

 light, and again that peculiar phenomenon of the ship sailing in a 

 luminous sheath was visible. The night was very dark, and it 

 was lightning vividly and incessantly ; the whole scene was eery 

 and weird in the extreme. I mention this case because it was 

 one of the most striking instances of general luminosity which has 

 come under my notice ; it appeared to be compounded of the two 

 forms I have already described. 



The third form of luminosity to be described consists of moon- 

 shaped patches of steady white light, which I have found to be a 

 very common phenomenon under certain circumstances. Next to 

 the occurrence of sparks and always accompanied by them, this 

 form of luminosity is most frequently seen, and does not appear to 

 be confined to any particular locality. I first observed it in the 

 Mediterranean, on the first night on which the absence of the moon 

 allowed it to be visible, and I have since found it to be no less frequent 

 in the Eed Sea, the Indian Ocean, the China Sea, and the Atlantic, 

 north and south of the equator. It is most commonly visible in 

 the wake of the ship, and consists of numerous round patches of 

 light, which might be mistaken for white-hot shot of various sizes 

 beneath the water at different depths. Sometimes, when deep 

 down, they were pale and of a whitish colour, with indistinct out- 

 line, and of large size, but when nearer the surface they were 

 smaller and more distinct, and assumed a pale greenish tinge. 

 They usually remained visible for 8 or 10 seconds, but sometimes 

 less. As these appearances were just such as might be presented 

 by the umbrellas of large Medusae, were such present and luminous, 

 I was strongly inclined at first to attribute them to this cause ; and 

 the fact that on one occasion (about a week after I left England) I 

 saw these moonlight patches in the Eed Sea on the evening of a 

 day on which the ship had passed through a shoal of Aurelise, led 

 me to attribute them to this cause. I supposed that the Aurelise, 

 struck by the screw, gave out their light under the excitation of 

 the blow, and floated away luminous and dying. But I was forced 

 to abandon this theory afterwards, for I have since many times 

 watched for floating Medusae before the light failed, and not seen 



YOL. rv. 2 l 



