liver Temperature. 299 
river and the dense fogs that hang over some large rivers, 
thinning out as they extend to the country round. These 
fogs arise from the opposite condition of things; the water, 
being several degrees cooler than the air, here performs the 
office of a condenser. Rivers after issuing from the gloomy 
and rarely sunlit ravines of a lofty range, carry with them 
for some distance in the plains the chilly temperature of 
their mountain courses. For instance, the Brahmaputra, 
after emerging from the eastern Himalayas into the full 
exposure of the valley of Assam, long retains its coolness, 
and the fogs and mists that form over its surface mark out 
its track for 50 miles or more. ‘This I gather from the 
observations of Dr Griffith on this river at Sadiya in 
September 1836. The temperature of the water during the 
whole month was, day and night, below the air, the difference 
in the means for the period being 12°-9 (F.). There is no 
space here to dwell on the manner in which the salubrity of 
a river-valley may thus hinge on the temperature of a river. 
It has occupied the attention of many, notably Fournet in 
the case of the Rhone, and the observations of Griffith will 
enable us later on to in some measure estimate one element 
of unhealthiness, namely, the rapid nocturnal air chill. 
Coming now to the hour of a river’s maximum tempera- 
ture, I may remark that but few observers seem to have 
made this a special subject of investigation, though several 
give sufficient data for an approximate estimate of it. Once 
again we look to Renou for guidance in this matter. From 
his hourly observations on the Loir, it appears that the 
water attained its highest temperature at 3 or 4 in 
the afternoon in summer, and at 2 or 3 in winter. 
For the Seine, he places the time of the maximum at between 
3 and 4, In connection with the Elbe’s tributary, the 
Saale, Dr Ule remarks that the water is often higher in the 
evening than at noon, and I would infer that in summer the 
maximum temperature is reached at an advanced hour of the 
afternoon. The Arve in summer, according to Professor 
Chaix, increases in temperature until 9 and 11 at night, an 
unusual feature, which may be due to the Alpine character 
of its basin. On the Thames I made numerous observations 
