292 Proceedings of the Royal Physical Soctety. 
Parana, near Rosario, in the midst of a southern summer, 
when he obtained a surface temperature of 80° (F.), whilst 
the thermometer in the shade registered 110°. 
Lastly, I give the results of my own observations on the 
Thames and its tributaries above Teddington Lock. They 
were made with a thermometer of the Sixe pattern supplied 
to me for the purpose by Messrs Negretti & Zambra, an 
ordinary thermometer being, however, used for ascertaining 
the temperature of the surface inch. Until I could thoroughly 
trust the Sixe instrument, the following apparatus was em- 
ployed as a check, which proved eventually to be unneces- 
sary:—Two minimum thermometers were fixed between two 
wire gratings, such as are used for pressing plants. The 
instruments were arranged crosswise, so that any jerk or 
other disturbance would be detected by the different readings 
of the indices. My apparatus was cumbrous, but in the 
workshop of a mechanician a handy means might be 
devised. 
My observations on the Thames were made in mid-stream 
between Teddington and Walton Bridge, and therefore above 
tidal influence. The results of twelve sets of experiments 
made between 1 and 4 P.M. in the warm season from May to 
September were as follows:—On the sunniest days, in depths 
ranging from 7 to 12 feet, the difference between the surface 
inch and the bottom varied from 1° to 1°5 (F.). When the 
sky was somewhat clouded the difference was only 0°°5. 
The current was very sluggish during this summer, and was 
not usually more than half a mile an hour. I may refer 
particularly to one of these days, as apparently affording 
very favourable conditions for surface-heating. It was a 
broiling hot June afternoon, with a cloudless sky, not a 
breath of wind, the current not half a mile an hour, and the 
air in the shade at 86° (F.). Four observations gave a con- 
stant difference between the bottom and the surface inch 
of 1°. The temperatures at intermediate depths varied a 
little in these experiments, the mean occurring sometimes 
at the middle depth, at other times above or below it. 
These differences between the surface and bottom tem- 
peratures apply, as I have said, to summer afternoons, and 
