25 
of Edinburgh, Session 1872-73. 
Temperature of the Water at Great Depths in Loch Lomond, as 
exemplifying that of the deep fresh-water lakes of Scotland 
generally. I afterwards communicated other observations made 
in the middle of April last, on the first approach of weather 
warmer than that of the preceding winter months; which, however, 
were unusually open and free from frost. The result was that 
between the middle of September 1871, to the middle of April 
1 872, in parts of Loch Lomond, varying between about 500 and 
600 feet in depth, there is constantly at the bottom a great sheet 
of water from 250 to 350 feet in thickness, the temperature of 
which remained steadily at 42°, whatever might be the temperature 
of the surface-water, or that of the air immediately over it. I beg 
now to supplement these observations very briefly with a few 
made since, in continuation of them. 
But allow me, in the first instance, to do justice to others who 
had previously made observations somewhat similar, and whose 
results were last year imperfectly, and some of them altogether, 
unknown to me. 
So early as 1767, Horace Benedict de Saussure made thermome- 
trical observations in the lake of Geneva, finding the temperature 
at 82 feet to be 55 0, 6, when at the surface it was 78°. This was 
in the middle of August. 
In 1774, Mallet and Pictet, in a deeper part of the lake, opposite 
the Castle of Chillon, found at a depth of 300 feet a temperature of 
51°, while that near the surface in August was 76°. This result, 
says Saussure, “ is very remarkable ; for 51° is two degrees and a 
half below the mean temperature of the earth at Geneva.” 
De Saussure afterwards extended his researches greatly. But, in 
the first place, not being acquainted with any available register- 
thermometer for such observations, he laboured to construct one 
which should retain, when hauled up, the temperature it had 
attained at the bottom. He at last succeeded in constructing such 
an instrument by using a thermometer whose bulb was an inch in 
diameter, surrounding it with a non-conducting coat of wax, resin, 
and oil three inches thick, encasing the whole in a wooden box, 
two-thirds of an inch in thickness, and securing the whole with 
tight iron ferrules. His instrument, which was thus a cylinder 
above seven inches in diameter, had the lamentable defect of 
vol. vnr. 
