464 PROFESSOR CHRYSTAL 



doubt of the correctness of the observation, because it was confirmed by another pair of 

 observations, one made almost exactly at the position above indicated, the other 250 

 vrards farther east. The latter gave on residuation a well-marked binodal seiche, the 

 former none that could be recognised. 



Eastern Trinode. — The best observation available places the southern end of the 

 Eastern Trinodal line 88 yards west of the calculated position. The uncertainty of 

 the determination, however, exceeds 120 yards; so that we cannot say for certain 

 whether the actual trinode is really west or east of the calculated position ; from 

 the shape of the normal curve we should expect a considerable divergence to the east. 



Middle Trinode. — Unfortunately, the observations for the determination of the 

 Middle Trinode were rendered useless by casual wind-disturbances. 



Western Trinode. — No observations of sufficient accuracy are available. 



PART IV. 



EFFECT OF METEOROLOGICAL CONDITIONS UPON THE 

 DENIVELLATION OF LAKES. 



General Character of the Seiches on Loch Earn. 



Owing to the comparatively regular shape of its basin and the fact that the depth 

 is considerable compared with the length, the seiches on Loch Earn are very regular 

 and very persistent. Also, probably because its longest axis is more or less parallel to 

 the paths of the major and minor atmospheric disturbances,* Earn is very rarely free 

 from seiches. During 1070 hours, from 10th August to 28th September, the Waggon 

 recorder at Picnic Point was almost constantly in action ; yet only 2|- hours of calm t were 

 recorded. During 1350 hours, from 12th October to 7th December, while the Waggon 

 recorder was in action at Lochearnhead, there were in all about 90 hours of calm. Of 

 these, 81 hours were made up by continuous stretches of 21 h , 37 h , and 23 h on 4th, 16th, 

 and 20th November. 



The greatest ranges observed in August and September were 79 mm., 66 mm., 

 73 mm., 55 mm., 55 mm., 63 mm., on 19th and 21st August and 3rd, 7th, 8th, and 

 9th September. Only one very exceptional range was observed between 12th October 

 and 7th December, viz. 55 mm. on 7th December. 



The range of the seiche at St Fillans is usually over 10 mm. A rough estimate 

 showed that during the 1070 hours of observation at Picnic Point the range of the 

 seiche was over 30 mm. during 214 hours ; and during the 1350 hours at Lochearnhead it 

 was over 30 mm. during 57 hours only. It follows that, whether we test by hours of 

 calm, by hours of excess over 30 mm... or by occurrence of exceptional ranges, the period 



* See my paper, " On the Theory of the Leaking Microbarograph, etc.," Proc. R.S.E., vol. xxxviii., p. 454 C1908). 

 t I.e. whole range of seiche less than 2 mm. 



