^38 Dr Robertson on tJie Luminosity of tJie Mediterranean. 
luminous appearance is but rarely met with during the winter 
season, compared to the frequency with which it occurs in the 
summer and autumn. I have never seen it when the wind 
blows fresh from a northerly point, or when the temperature 
of the air is low; nor have I remarked it in any great de- 
gree, but in calm weather, when the temperature of the air was 
liigh, and especially as the wind was changing towards a south- 
erly point, and I liave always observed that this luminous matter 
was emitted most strongly immediately preceding the fall of 
rain or an overcasting of the sky, shewing a disposition to the 
formation of that meteor. Upon the whole, I am of opi- 
nion, that this appearance of light on the agitation of the 
waters of the Mediterranean, is somehow or other connected 
with evaporation, and that it is occasioned by the rapid evolu- 
tion of the electric fluid in that process ; that probably it rare- 
ly depends on phosphoric matters existing in the waters ; and 
after repeated and careful observations by myself and others, I 
have never been able to trace its appearance as depending on 
the existence of insects, nor could I ever perceive any thing pe- 
cuhar in the sensible qualities of water taken up in such circum- 
stances, or that was different from water of the sea when it did 
not give out this luminous appearance. Moreover, as this ap- 
pearance is only perceived on the agitation of the water, it 
would therefore seem that the luminous matter, whatever it may 
be, does not depend on a matter that is merely blended or mix- 
ed in the water, as in such a state* its appearance would be 
equally manifest in a calm, as when there is a gentle ripple on 
the surface ; and it must have been equally discernible in all 
conditions of the weather ; and what I consider as tending to 
corroborate my opinion, that the electric fluid is generally the 
cause of this shining appearance, an ingenious Greek physi- 
cian informed me that he found it always extremely difficult to 
accumulate the electric fluid in his apparatus on such occasions. 
In support, however, of the opinion I entertain of the cause of 
the luminous appearance of the Mediterranean Sea, I may ad- 
duce the observations of M. Golbery on this question, as given 
in the second volume of his Voyage to the Coast of Africa. At 
the same time, I do not mean to deny that this appearance may 
be occasionally produced by other causes. 
Isle of Thanet, Margate, March 1. 1819- 
