298 Proceedings of the Royal Physical Society. 
At such a time the water is usually several degrees 
warmer than the air. In the case of the Thames I have 
frequently found at dawn a difference of from 6° to 12° (F.). 
Renou, according to his table of hourly means for the Loir, 
noticed on the average a difference of 10° or 12° (F.) in 
summer and considerably less in winter. In the instance of 
the Saale, near Halle, Dr W. Ule places the difference 
between the means of the 6 to 7 A.M. air and water observa- 
tions for the year at 3°95 (C.), or 7°1 (F.). During March 
on the Lower Nile, Dr Marcet found the river at sunrise to 
be on the average about 14° (F.) warmer than the air. On 
the Santa Cruz river in Patagonia, Captain Fitzroy observed 
in April that the water at daybreak was 16° (F.) warmer 
than the air. 
In the tropics, also, the river-water is usually warmer at 
sunrise. On the Senegal in August Captain Borius ascer- 
tained that on the average the river was at 6 a.m. 4°°3 (F.) 
warmer than the air. Schomburgk on the Essequibo found 
the water to be generally 8° to 10° (F.) warmer at 6 AM. 
Humboldt on the Orinoco noticed in April a difference of 
about 7° (F.). Wallace on the Rio Negro noticed the 
average difference in September to be 9°-4 (F.). 
These facts may appear to unnecessarily illustrate a well- 
known feature of river temperature, but I have quoted them 
to give point to another familiar character of rivers, namely, 
their “smoking” or steaming in the early morning. The 
moisture arising from the river is condensed by contact with 
the cold morning air and forms a white vapour that rolls 
about on the surface of the water. This phenomenon came 
very conspicuously under the notice of Captain Fitzroy 
during the ascent of the Santa Cruz river in Patagonia, when, 
as before observed, he found the water 16° warmer than the 
air. It is to be seen on most fine summer mornings on the 
Thames at Kingston, and is especially remarkable during 
severe winter weather in the case of small rivers that, owing 
to their abundant spring supply or to their underground 
courses, do not freeze, such, for instance, as the Iton in 
France and the Wandle in England. 
The distinction must: be made between the steaming of a 
