52 Proceedings of the Royal Physical Society. 
1843, indicate that in February and March the river-water 
flows through the Passes into the Gulf at the temperature it 
possesses opposite New Orleans. On 24th February, the 
temperature of the river-water in the South-West Pass was 
42°-5, which is about the mean temperature for February of — 
the river at New Orleans; the Gulf-water, a few miles out, 
was 56°5, or fourteen degrees warmer. On 1st March the 
river temperature in the North-East Pass, off Balize, was 
42°-5, which was also the temperature of the river four days 
later at Fort Jackson, thirty miles above. Assuming that the 
river does not change much in temperature between New 
Orleans and the Gulf, I have arranged the following table :— 
TABLE XI, 7 
Comparison of the Temperature of the Mississippi at its Mouths 
with that of the Surface-Water of the Gulf of Mexico. 
Locality. | February. May. August. | November. 
Mouths of the Mississippi, . 43°0 66°5 84:0 60°5 
Gulf of Mexico, . : ; 70°0 78°0 83°5 72°0 
Mississippi difference, . —27°0 —11°5 +0°5 —11°5 
The data in this table relating to the Gulf of Mexico refer 
to the northern third of the Gulf, and have been taken from 
the same Admiralty chart of surface-temperatures that was 
employed in comparing the Mediterranean and the Nile. 
We here notice that the waters of the Mississippi are very 
much colder than the surface-waters of the Gulf in winter, 
considerably colder in the spring and autumn, and about the 
same temperature in summer. As far as temperature is 
concerned, during the greater part of the year there could be 
but little interchange between the faunas of the Mississippi 
and the Gulf of Mexico; and from the circumstance of such 
an interchange being confined to the summer, it may be 
inferred that the migration from one region to another 
cannot be on an extensive scale. The Mississippi during 
recent geological times has thus presented a great contrast 
to the Nile; the similarity in thermal conditions will have 
