422 MR E. M. WEDDERBURN 



evidence, the water is in a well-marked stratified condition, and there is a more 

 or less well-marked surface of separation between the upper layer of warm water 

 and the lower layer of cold water. There is an arrangement similar, say, to a layer 

 of oil resting on a layer of water. In this condition it is possible to have an 

 oscillatory movement in the lower layer independent of movements progressing 

 in the upper layer. The possibility of such oscillations may be readily verified 

 experimentally, and rough calculations of what should be the period of such an 

 oscillation in a loch support this theory. 



The observations for 1903 have been sufficiently dealt with by Mr Watson. In 

 1904, during a considerable part of the month of August, observations were made 

 every two hours, day and night. The observations made in July and August are 

 shown graphically in fig. 8. The number of the observations in August tends to 

 make the curve obscure owing to the embroideries, but beginning on 23rd July there 

 is a pretty well-marked oscillation at 100 feet, and even earlier there are evidences 

 of a periodical movement. The long period changes prior to that are attributable 

 to wind. For instance, on 11th and 12th July there was a strong N.E. wind which 

 brought all the warm water to the Fort Augustus end of the loch, and from the 

 18th onwards there were N.E. winds, which again show their effects at 200 feet. 

 The evidence of the influence of the wind at these depths is in accordance with 

 what has already been said about the warm water blown to one end of a loch 

 being quickly spread through the whole body of water if the Sprung 'schicht has not 

 made its appearance. The change of wind on 30th July gave rise to an unstable 

 condition of the isotherms (the normal direction for the isotherms is of course 

 horizontal). In this case the result of this unstable condition was the beginning 

 of a temperature seiche. Thereafter, as the Sprung schicht becomes more well marked, 

 the changes in the lower layers go on independently of the changes in the upper 

 layers, which are caused directly by the action of wind, sunshine, etc. Sudden 

 changes of temperature such as are shown on 8 th and 14th August at 50 feet are 

 easily explained on this theory. The temperature seiche for practical purposes may 

 be looked on as an oscillation of the Sprung schicht. When the Sprung schicht 

 passes any particular point in the loch there will be a rapid change of temperature 

 at that point, for above the Sprung schicht there is warm water, and below, the 

 water is comparatively cold. In no other way, so far as I am aware, can these 

 rapid changes be explained, and to my mind they form one of the strongest supports 

 of this theory. Fig. 9 shows a record obtained by means of the electrical recorder 

 on 17th August with a thermometer at 100 feet. It shows that in a quarter of an 

 hour there was a rise in temperature at 100 feet of 8 - 3°, and though such sudden 

 changes were by no means usual they are not unparalleled. On the same date at 

 150 feet the temperature observed by means of mercury thermometers rose from 

 43*6° to 53*5°, or practically 10°, between 2 p.m. and 4 p.m. The distinctness of the 

 surface of separation will determine the rapidity with which these changes take place. 



