350 Action of Coloured Light on Carp. [June, 
deficient in this ; and with our growing wants for drainage 
and for the prevention of floods from rivers, that deficiency 
is a practical evil. We will instance what we have in mind. 
The French inundations of 1875 will be long remembered, 
when almost the whole of Languedoc was under water, and 
Toulouse was nearly destroyed. Now we know that in pre- 
historic times this overflow of the Garonne was prevented 
or greatly lessened by lakes now filled up, and that at the 
present day the floods are thus vastly more violent and 
destructive than formerly. The practical lesson is obvious. 
The foundation of a geographical map is its outline, and 
that consists chiefly in the delineation of its waterways ; a 
distinct acquaintance with this branch of the subject is a 
fundamental element of geographical knowledge. As Mr. 
Saunders says in his Introduction, “ in tracing a water- 
course from its source to its final outfall, it is found that 
some outfalls dispose of the waters of small areas with 
simple systems of watercourses ; others are the drains of 
large areas with a great complication of watercourses not 
easily unravelled.” It is just in this systematic examina- 
tion that the Palestine Survey and its Map are so admirable, 
and that our Ordnance Surveys- and the best contoured maps 
are so defective ; a serious hindrance, and an impediment, at 
every Sanitary Board in the kingdom, when attempting the 
drainage of its district ; for a “ water-parting,” be it remem- 
bered, may be as much a simple ridge, or a mere swelling of 
the ground between two of the smallest streams, as the 
natural division of the waters of a hemisphere ; one of the 
lessons practically taught by the Map of Palestine and its 
u Introduction.” 
VI. NOTES OF EXPERIMENTS ON THE 
ACTION OF COLOURED LIGHT ON CARP. 
By A. Anthony Nesbit, F.C.S. 
<L± 
r is well known that fish alter in shade when the colour 
of the bottom of the water in which they live is altered, 
becoming darker when the bottom is darker, and lighter 
when the bottom is lighter. For the purpose of ascertaining 
whether this alteration is due only to the quantity of light — 
