G26 TRANSACTIONS OF SECTION A. 



3. Temperature Conditions in Scottish Lochs. 

 By E. M. Wedderburn, F.R.S.E. 



Very complete obaervations have been made iu connection with the Scottish 

 Lake Survey in Loch Ness, which is a lake of the Tropical class, with a length 

 of 22^ miles and a maximum depth of 750 feet. Numerous observations have 

 also been made in Loch Garry (Inverness-shire), which is a lake of the Temperate 

 class, from February to July 1908. The main basin of this lake has a length of 

 about 4 miles and a maximum depth of about 220 feet. 



These observations indicate that iu winter, at which time the lakes are of 

 nearly uniform temperature throughout, the etlect of winds blowing along the 

 surface of the lake is to produce a circulation of the whole of the water in the 

 lake ; and further, that in summer, when the water in the lake has had a con- 

 siderable accession of heat, mainly by isolation and conduction, the effect of winds 

 blowing along the surface is to produce a discontinuity in the temperature 

 gradient of the lake at a depth dependent on the particular circumstances of 

 each case. 



An experimental investigation in which a long glas.s trough was used to 

 represent the basin of a lake, and diflerence in density of the liquid was used to 

 imitate difference of temperature, indicates that the discontinuity is produced 

 by the return current, induced in the lake by the wind-produced surface current 

 becoming localised and taking place at a relatively small depth when the increase 

 of temperature at the surface has produced considerable differences in temperature 

 in the lake. 



Whenever the discontinuity is pronounced the layer of water above it behaves 

 independently of the lower layer. The surface layer shows changes which can be 

 directly traced to the action of the wind driving the warm water to one end of 

 the lake. The lower layer shows great variations of temperature, and it has been 

 satisfactorily demonstrated that these variations in temperature are due to what 

 is called a temperature seiche in the lower liquid, with period dependent, inter alia, 

 on the depth of the discontinuity and the difference in density between the upper 

 and lower layers. The amplitude of this temperature seiche in Loch Ness was at 

 times from loO to 200 feet, and the period was about three days. In Loch Garry 

 the period of oscillation was about twelve hour.s. 



Observations were made in Loch Garry and Loch Ness in 1908 with Ekman's 

 propeller current meter. 



Owing to the experimental dif!icultie.« encountered, and the danger of making 

 lengthy observations in high winds, the observations have not been so complete as 

 was hoped for, and must be looked upon as preliminary. They show, however, 

 that early spring currents are appreciable at very great depths, and that later in 

 the year the currents are chiefly felt near the surface. 



The author has been criticised for generalising from Scottish Lochs, which are 

 narrow and situated in glens in wliich the wind can only blow with anv force 

 in one of two directions, to broad and open lakes. It seems unlikely, however, 

 that the phenomena observed in these lakes do not have their counterpai't in all 

 lakes, except in those which are very shallow and are kept of uniform temperature 

 from top to bottom by the action of winds. In Continental lakes there are no 

 observations known to the author which are sufficient to detect oscillations similar 

 to those noticed in Scottish lochs. 



4. The Constants of the Lunar Libration. By F. J. M. Stratton. 



A reinvestigation of the heliometer observations of Mosting, A., made by 

 Schliiter at Konigsberg in the years 1841-43, has been undertaken in the hope 

 of reconciling the conflicting sets of constants for the lunar libration given by 

 Drs. Franz and Hayn. 



The expense of the work of checking the computations has been borne in 

 part by a grant from the Royal Society. 



