252 TRANSACTIONS. 
some degree upon the temperature of the tidal water, while that 
of the river is wholly unaffected by it. It may perhaps be asked 
why the mean annual temperature of the air in my observations 
should be 52°5°, while in those of Mr Lewis it is only 47°7°, and 
the mean temperature of the water 48°5°, as compared with 46°. 
But this admits of an obvious explanation. For one thing, there 
were no observations taken by Mr Lewis in April, May, and the 
greater part of June, while mine included these months, and 
another thing to be taken into account is that my observations 
were taken invariably about noon, when the heat of the day was 
approaching its maximum, while those of Mr Lewis were taken at 
all hours when the tide was up. 
I regret that I have not been able to procure a sufficiently 
extended number of observations of the temperature of Lochrutton 
Loch to be of much value, but by the kindness of Mr Beck and Mr 
Lindsay I got observations made from the 8th to 19th August, 
and from 13th September to lst October, with the following 
results : 
Means. 
Air. Water. Difference. 
From 8th to 19th August... .. 583 61-1 +2°8 
», 13th September to Ist Ouiehen ne SOASE 55'2 +0°9 
From this we may probably infer that during at least the autumn 
and winter months, and possibly in summer also, the temperature 
of the Loch is, as a rule, in excess of that of the air. But the 
observations are too limited in number to warrant any decided 
conclusion being founded upon them. 
Il. Seaweeds. By Mr JosEpH WILSON (late Secretary). 
Seaweeds form by far the largest section of the Algz, which 
is one of the three great classes into which the thallogenous plants 
are divided. They are most abundant in the tropics, and many 
thousand species have been found in the waters surrounding the 
British Isles. These aquatic plants vary very much in size, form, 
texture, and colour, Some species are entire and coriaceous, 
others branched and filamentous ; some are flat, with or without a 
midrib ; others are round, and in some instances measure several 
hundred feet in length. One characteristic feature of all is that 
they have no true roots, but absorb their food instead from the 
medium in which they exist. 
