38 Proceedings of the Royal Physical Society. 
the river, as indicated in the day’s means, must be some 
eight or nine degrees cooler than the air. My example, 
however, is not an imaginary one, it is that of the Nile 
in June at Beni Hassan as given in Table I.; and in the 
columns of the same table, it will be observed that the 
difference of the air and water means, estimated from about 
six daily observations for the air and two for the water, 
is 8°4, All the elements for determining approximately the 
relative positions of the air and water temperatures by 
means of the sunrise observations will be found in Table XV. 
To enlarge on the subject here would be to open up the 
complicated matter of the daily range, which can only 
be very cursorily treated in this paper. 
The relative coolness of the river to the air in summer 
being proved alike by the comparison of the daily means 
and of the sunrise observations, I pass on to consider yet a 
third plan, by which this characteristic feature of the river 
can be established. In this method I make use of the 
isothermal charts, placed at our disposal through the labours 
of Dove, Mohn, Buchan, and others, Laying aside for a 
time the air observations, and relying only on those of the 
river, I appeal to the meteorologist. The chart for July, as 
constructed by Buchan, will answer the purpose. According 
to Hay’s observations at Abu Simbel, in latitude 22° 20’, 
during July 1831, the mean temperature of the Nile for that 
month was 78°°8, and of the air 92°-1. Now the isotherm of 
95° for July is drawn across the Nile in about latitude 23°. 
From this it follows that I have not exaggerated the relative 
coolness of this river, with respect to the air, in a locality 
that possesses the highest mean monthly temperature on 
the globe. Within the borders of the tropics, the Nile is, in 
July, about thirteen degrees cooler than the air, as indicated 
by the observations of Hay, and about sixteen degrees as 
implied by the isothermal chart. Descending the river to 
the locality between Tel and Beni Hassan, latitude 27° 37’- 
27° 53’, we learn from the data supplied by Hay, that the 
mean temperature of the Nile for July 1830 was 78°4, and 
of the air 86°3. The Nile is crossed by the isotherm of 85° 
for July in about latitude 28° 30’, and allowing for the 
