290 Proceedings of the Royal Physical Society. 
ture of the Rhine at the surface and at a depth of 5 metres. 
Professor Fournet is my authority for this statement. In 
1851 M. Renou, in the case of the Loir at Venddéme, placed 
the difference of the mean surface and bottom temperatures 
at a few hundredths of a degree Centigrade, the Loir in this 
locality being 3 to 5 metres deep. It is observed by 
Lieutenant Marr, in the report by Captain Humphreys and 
Lieutenant Abbot on the Mississippi, that the difference 
between the temperature of the water at the surface and at 
the bottom was usually so slight as not to be observable 
with the common thermometer. Unfortunately the heads of 
this famous survey regarded the systematic observations 
made for two years and more on the temperature of the 
river as too purely scientific in their interest to embody in 
any detail in their report. Coming to the Weser, we find 
similar evidence in the paper of Herr von Freeden, wherein 
he gives the results of ten years’ observations on that river 
at Elsfleth, near Bremen. On account of the mixing of the 
water, the temperature, he says, is the same in all depths. 
At this locality the tidal influence is strong, just as we find 
in the case of the Thames at Greenwich, the scene of a 
famous series of observations covering thirty-three years, 
which have been discussed by Sir George Airy. 
Amongst the most important observations bearing on this 
subject, which have hitherto come under my notice, are those 
made in August 1876 by Captain E. Borius and Dr Louvet 
during their ascent of the Senegal to the cataracts of Felou, 
about 500 miles from the sea. In connection with river 
temperature, I shall have frequently to refer to this expedi- 
tion, which was organised mainly for the purpose of making 
meteorological observations on the river. Its promoter was 
Dr A. Borius, who was especially anxious to supply infor- 
mation concerning tropical rivers, the lack of which had 
been recently deplored by M. Renou. These systematic 
observations, which go to show that in respect of tem- 
perature with depth the Senegal presents no exception to 
‘the general rule, were made usually several times daily 
between 6 AM. and 8 P.M., both at the surface and at a 
depth of 5 metres, which was half the usual depth of the 
