TRANSACTIONS OF SECTION E. 445 



sea-level at Mombasa in September 1907 by Mr. Welldon (Egyptian Survey), 

 and by Mr. Landon (Egyptian Irrigation Service) as 617-197^1. in February 

 1909. Both heights are apparently obtained from a good bench-mark at Butiaba, 

 which is connected both by spirit-levelling and by trigonometrical heights to 

 Entebbe, hence by two years' comparisons of tide-gauges with the railway 

 levels at Port Florence, and hence to Mombasa by the railway levels. 



There is no evidence to show the cause of the discrepancy between the two 

 values for the Butiaba tide-gauge, so a mean of the two values has been taken. 



We get, therefore : mean height of Lake Albert, May to July 1908, 

 2,027-9 feet; height of Lake Albert Station, 2,03324 feet. 



The probable error of this latter value may be roughly taken at ± 4'7 feet, 

 hence height of Ruwenzori (highest point), 16,801-3 ± 5-3 feet. 



In a paper published in the Royal Geographical Society's Journal for March 

 1907 the following figures were given : Position, 0° 23' 0" N. lat., 29° 52' 20" 

 E. long.; height from the best trigonometrical data at the time, 16,619 feet; 

 height as determined barometrically by the Duke of the Abruzzi, 16,814 feet. 



This trigonometrical height was arrived at from observations taken under 

 rather unfavourable circumstances from the south, the length of the rays being 

 100 miles and over. 



The discrepancies in position between the new and old values are unim- 

 portant; the new values are subject to correction when the arc centre is finally 

 worked out. 



The discrepancies in height are due to uncertainty as to the refraction, which 

 has not been taken into consideration in the above probable errors. 



The coefficient of refraction may easily be 0'005 out, or even 001. The effect 

 of such an error varies as the square of the distance from which the observations 

 were taken, and trigonometrical observations taken from a distance of 100 miles 

 cannot be considered' more accurate than careful barometric heights, such as 

 those of the Duke of the Abruzzi. 



Personally, I am inclined to think that 0060 is too low a value for the co- 

 efficient for Captain Jack's observations, and I should give the height of 

 Ruwenzori as 16,780 feet, with a probable error of 20 feet. 



FRIDAY, SEPTEMBER 1. 



Joint Discussion with Section C on the Former Connection of the Isle of 

 » Wight with the Mainland.— Bee p. 384. 



The following Papers were then read : — 



1. Constructive Waterfalls. By Professor J. W. Gregory, F.R.S. 



Waterfalls are among the most effective agents in deepening river valleys, 

 and thus lowering the land. Their influence is usually regarded as solely destruc- 

 tive ; but they may, under some conditions, be constructive and act as agents of 

 deposition instead of denudation. This action is well illustrated by some water- 

 falls in Dalmatia, Bosnia, and Herzegovina. 



The Kerka Falls in Dalmatia, ten miles from Sebenico, illustrate a simple type 

 of constructive waterfall. They are due to a barrier of calcareous tufa, which 

 the Kerka River has buil't up across its valley. The barrier is 130 feet high ; the 

 river is subdivided into many channels, and each falls in a succession of cascades 

 over the tufa terraces. Above the barrier is a plain of alluvium, and a short dis- 

 tance up-stream the river flows from two lakes, one of which is eight and a half 

 miles long. The lakes have been formed by the tufa dam, and as this barrier 

 is being raised in height with its advance down-stream, the lakes are being 

 increased in size owing to the action of the waterfall. 



The Topolie Falls on the Upper Kerda River afford a clear illustration of the 

 construction of a tufa dam by a waterfall. The fall is about 70 feet in height, 



