335 



XXXIV. — Note on two Streams flowing from a common Source in 

 opposite Directions. By Professor H. Hennessy, F. E. S. 



[Read June 22, 1868.] 



The peculiarities of river watersheds appear to possess much interest 

 for geographers, and have frequently excited discussion in recent 

 times. * I may, therefore, be excused for attempting to make a trifling 

 contribution to the facts already collected. Amidst the group of 

 mountains to the south of Dublin, two small streams arise, one of 

 which is traceable from the Dodder, through Eockbrook, up to Glendoo ; 

 the other is traceable from its junction with the Dargle Eiver at St. 

 Valerie, up through Glencullen. Both streams flow from the same 

 point, which is precisely at the highest part of the axis of the ravine, 

 one end of which is denominated Glencullen and the other Glendoo. 

 At the point in question, there is a hollow or pot constantly full of 

 water, which is received laterally from a brooklet that rises much 

 higher amidst the boggy slopes on the sides of Criiagh and Glendoo 

 Mountains. The parting of the streams is not shown on the Ordnance 

 Maps, but there seems to be a rude indication of its existence in 

 Eocque's Map of the County Dublin, published during the last cen- 

 tury. No topographical writer appears to have hitherto noticed the 

 phenomenon, and it has thus seemed to me desirable that it should be 

 systematically placed on record. The partings of small temporary 

 streams frequently arise after heavy rains, but, as in this case, the ob- 

 servations were all made during the prevalence of dry weather, the 

 phenomenon may be considered as comparatively permanent. I visited 

 the spot three times, and on the last occasion (June 17) I found almost 

 all the watercourses which I crossed on the sides of Tibradden Moun- 

 tain perfectly dry, while the turf was everywhere hard. No rain had 

 been recorded in Dublin since the 3rd, and then it had fallen in a small 

 quantity ; while a shepherd whom I met near the bifurcating streams 

 assured me, that for the preceding three months the dryness which pre- 

 vailed in the locality was quite unusual. In company with Mr. J. 

 0' Kelly, of the Geological Survey of Ireland, I minutely examined the 

 circumstances of the water parting. We verified the precise point of 

 bifurcation which I had previously detected in the pot already alluded 

 to, by scattering in the water some fragments of moss which had 

 nearly the same specific gravity as the water itself ; after a short inter- 

 val some fragments were carried N. W., towards the Dodder, while 

 others were carried S. E., towards Glencullen and St. Vallerie. The 

 appearance of the ground exhibited no trace of artificial cutting or 

 embanking that might give rise to the bifurcation, while the rushes and 

 moss which surround the diverging streamlets seem to have been long 



* See the "Athenaeum," volume for July to December, 1863, pp. 19, 59, 83, 113, 

 248, 578, 652, and 657; also the volume for January to June, 1866, pp. 367, 398, 

 499, 564, 636, 



