416 MR E. M. WEDDERBTJRN 



surface temperature for August (approximately). One would then expect a change of 

 type in the curve for September, as the air temperature has fallen considerably below 

 the surface temperature, and in fact there is a remarkable change of type. The water 

 cools off in an entirely different manner from that in which it gained in heat. It cools 

 gradually from the top, and not uniformly through its whole bulk. This appears more 

 clearly from fig. 4, which represents the mean temperature changes at the surface, 50, 

 100, 150, and 200 feet respectively, throughout the year. This diagram shows that 

 whereas the surface layers begin to cool off in August, the lower layers go on getting 

 warmer for another month or six weeks, and the loch is in fact gaining heat for a 

 time after the surface begins to cool off. The highest temperature at 700 feet, as 

 already mentioned, was recorded in the middle of November, or about three months 

 after the loch began to show signs of losing heat, indicating that water at this depth is 

 below the region of disturbance by wind and currents, and that the temperature changes 

 are here the result of conduction. 



It is difficult to form a definite opinion as to the cause of this change of type. 

 There is nothing in the difference in the weather in spring and autumn which can 

 explain this. The actual state of affairs in autumn is not very clearly shown by the 

 typical curves. The curves in fig. 1 show clearly that there is, to a certain depth, 

 water at a uniform temperature, and that thereafter the water gets gradually colder. 

 AVithout referring at present to the cause of this, it may be stated that in reality there 

 is a layer of water at uniform temperature of gradually increasing thickness. Where 

 this layer ends there is an abrupt change of temperature, and below this point, which 

 is called the Sprung schicht, the water gradually decreases in temperature. The 

 Sprung schicht, however, oscillates up and down, so that when the mean temperature 

 is taken over a considerable period, the effect of this oscillation is to take away from 

 the typical curves all indication of a sudden and abrupt change of temperature. 



III. Influence of Wind. Sprung schicht. 



The appearance of the Sprungschicht is, to my mind, due to the effect of the wind. 

 The general effect of wind on a fresh- water loch is now well known.* The water on 

 the surface is blown along by the wind and accumulates at the windward end of the 

 loch. The place of the water so transferred to the windward end has to be filled. Con- 

 sequently a " return current" is set up and water from the deeper parts of the loch rises 

 to the surface. Thus there is always a process of circulation while there is wind blowing. 

 In a loch like Ness, where the water never falls below the maximum density point, it 

 is always warm water which is blown along the surface and relatively colder water 

 which rises to take its place, with the result that there is always a greater quantity 

 of warm water at the windward than at the lee end of the loch. Fig. 5 shows an 



* In particular, see Sir John Murray's paper " On the Effects of Winds on the Distribution of Temperature in 

 the Sea- and Fresh-water Lochs of the West of Scotland," ticottish (reograpliical Magazine, July 1888. 



