754 DR E. M. WEDDERBURN AND MR A. W. YOUNG ON 



vertical component is ±10° or less, 28 in which the vertical component is less than 

 ±20°, and 33 in which it is less than ±30°. No means were available of testing the 

 zero of the scale for measuring the vertical components, and it may be that there was 

 a considerable zero error ; but it is thought that the error in the observations could 

 not exceed ±30°. 



Excluding observations which do not show a vertical component of more than 

 ±30°, and including one observation on 22nd August, there are only nine occasions 

 on which a distinct vertical component was observed, out of a total of 179 observa- 

 tions, viz. : — 



6th August 



h 



17 



m „ 



20-5 metres + 90 



2 - 2 cms. 



7th 



)> 



12 



30-15 , 



, +86 



2-9 „ 



11th 



>) 



18 



50-10 , 



, -67 



2-8 „ 



>j 



M 



19 



30-10 , 



, -82 



2-5 „ 



>) 



»> 



22 



5- 3 , 



, -64 



2-2 „ 



14th 



)) 



19 



55-7 , 



, +64 



4-8 „ 



22nd 



J> 



15 



22-20 , 



, -47 



2-4 „ 



29th 



)) 



12 



55-15 , 



, -47 



27 „ 



)» 



>> 



18 



50-10 , 



, -40 



2-4 „ 



Considering, first, the observations which show an upward current, the tempera- 

 ture observations on 6th and 7th August are not such as to suggest an explanation, 

 save that at about the time of the observation there was a slight upward movement 

 of the isotherms. On 14th August the upward current is so well marked that it 

 can hardly be supposed that the record is due to instrumental error ; and yet the 

 occasion is eminently one on which a downward current might have been predicted. 

 For at the time of observation the discontinuity was in process of making a very 

 rapid descent, as is seen in fig. 8. The observation was made at a depth of 7 

 metres, whereas the depth of the discontinuity was at least 10 metres, and the only 

 suggestion one can offer is that the rapid descent of the discontinuity caused eddies 

 in all directions. 



Turning to the downward currents on 11th August. There had been easterly 

 winds all day, and these became stronger about 18h. The discontinuity was well 

 below 10 metres, and the thermograph which was recording at this depth showed 

 uniformity of temperature. There seems little reason to doubt but that the observa- 

 tions show a steady downward motion of the water at the lee end of the loch, prior 

 to the formation of the return current. 



The downward current on the 29th is also easily explained by the fact that a 

 strong easterly wind was blowing on that date, and reference to fig. 10 will show 

 that the isotherms were rapidly falling. The horizontal component of the currents 

 was mainly westerly, indicating that the observation was made within the return 

 current. The return current seems to have been very near the surface. The observa- 

 tion at 2 metres shows an easterly current directly due to the wind, but at 5 metres 

 the current is from the north, as if the return current was beginning to form. 



