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Proceedings of the Boyal Society 
mean of the two is a nearly straight line which almost coincides 
with the line of 50°. The number of observations made use of in 
drawing the winter curve was much greater than that available for 
the summer line, a fact which possibly accounts for the mean 
between the two not being parallel to the axis of x. 
The mean rise or fail of temperature going along the central line 
of the Firth at- any time during the year is 0°*07 per mile. In 
April and October it is much less ; in December and August much 
more. 
A great deal has to be done in the way of studying the effect of 
shallow water, and of the proximity of land in modifying the tem- 
perature in cross sections of the Firth. From what has been done 
already, it appears possible that if a large number of temperature 
observations could be taken at different parts of the I 'irth during all 
seasons for a few years, data would be procured which might render 
the thermometer a valuable aid in determining the position of a 
vessel at night or in a fog. 
At present (April 1885) the temperature of the River and Firth 
of Forth is in a position something like the following 
Loch Ard, the principal source of the Forth proper, is at a tem- 
perature of 42° *5, so far as regards the water in the six feet nearest 
the surface at any rate. The shallow river issuing from the loch 
and flowing slowly over a stony bed was, on the day of my visit last 
week, of the temperature 45°. At Kippen, 16 miles farther on, it 
was 47° '3, and some of the small tributary burns were as high as 
49° -5. At Gargunnock, 7 miles from Kippen, the temperature of 
the water was 46 c, 6. The Teith, which is a larger river than the 
Forth, and joins it near Stirling, had a temperature of 42° ’3 at 
Callander, which was reduced to 42° at Doune, 9 miles farther on, 
and 6 miles from the junction. This evidently shows that the lochs 
that feed the Teith — Yennachar, Katrine, and Lubnaig — must be 
colder than Loch Ard : but I had not time to ascertain this by a 
visit. At Stirling the river had the temperature of 45°, which 
gradually fell to 43° *5 at Kincardine; and the whole Firth, from 
there to beyond the Isle of May, is at a temperature not more than 
half a degree different from this. This uniformity will probably not 
last for a week. It is a state of matters represented by the upward 
sloping winter-temperature curve being in a horizontal position 
