i86 



NATURE 



[Dec. 25, 1879 



A TIDAL PROBLEM 



THE so-called seiches, or alternate flux and reflux of 

 water in the Lake of Geneva and other bodies of 

 fresh water, have, as our readers know, formed the 

 subject of an interesting study during the past decade by 

 Dr. F. A. Forel, of Morges, near Geneva. Small local 

 tides are constantly noticeable there, the difference 

 between ebb and flow varying from a few centimetres to 

 2 metres. Their cause is to be traced to the wind, 

 variations in atmospheric pressure at the extremities 

 of the lake, &c. Dr. Forel, as the result of his in- 

 vestigations has established a formula by means of 

 which the duration of a local ebb and flow can be 

 determined — not only for the Lake of Geneva, but for 

 any lake — when its average depth and its length are 



known. The following is the formula T = 2 — , in 



Sfh 



which L denotes the length of the lake, /; its average 

 depth, and g the acceleration of gravity. This formula 

 gives for the Lake of Geneva, which has a length of 



73 kilometres, a duration of tide of 13 minutes; a figure 

 coinciding with the fact. 



The law thus established by M. Forel has recently 

 received an interesting application in solving a problem 

 which has puzzled travellers and philosophers for over 

 2,000 years, viz., the explanation of the currents in the 

 narrow straits of Euripus, where the famous five-arched 

 bridge of Egripo joins the Island of Eubcea to the main- 

 land of Greece. The currents sweeping below the bridge 

 are so violent that mills are kept in operation by them, 

 but they are noted for the changes in direction which 

 occur from four to fourteen times daily. Tradition 

 relates that Aristotle, in despair at his inability to explain 

 this phenomenon, threw himself from the bridge into the 

 water. 



A comparison of the large number of observations 

 made upon this strange tidal movement shows that there 

 are two distinct periods : that in which there are but 

 four changes of direction or two tides in a lunar day of 

 24 hours and 50 minutes, and that in which these tides 

 number from eleven to fourteen daily. This latter pheno- 

 menon is observable invariably at the quadratures of the 



moon. M. Forel, in his explanation, shows that the 

 regular ebb and flow twice a day in the former period is 

 due to the tidal movement of the yEgean Sea, which is 

 then at its maximum. The increase in the number of 

 tides daily becomes manifest, however, when the tidal 

 force of the ^Egean is at its minimum, viz., at the quad- 

 ratures, and must be owing to some other force more 

 powerful than the minimum but less powerful than the 

 maximum force of the ^Egean tide. This force is found 

 in the local tides or seiches of the Gulf of Talanti to the 

 north of the straits, which is so shut in by land that 

 it can practically be regarded as subject to the same 

 laws as the lakes of Switzerland and other countries. 

 This basin is 115 kilometres long, and is from 100 to 200 

 metres in depth. Applying these figures to M. Forel's 

 formula, the ebb and flow in the Gulf of Talanti would 

 be for 100 metres, 122 minutes ; for 150 metres, 100 

 minutes ; for 200 metres, 86 minutes. The eleven to 

 fourteen currents observable daily at Euripus during the 

 quadratures last from 103 to 131 minutes. This shows 

 so striking a conformity with the theory advanced by the 

 Swiss savant, that we can but consider this problem, which 

 so vexed the ancients, as fairly solved. 



Dr. Forel asks intelligent visitors lo the locality to 

 verify his interpretation by attending especially to the 

 following points : — 1. Ascertain the exact duration of the 

 flux and reflux of the Euripus, and determine its normal 

 rhythm. 2. Ascertain if, as in the seiches of the Lake 

 of Geneva, the amplitude of the flux and reflux of the 

 irregular current is stronger in bad weather than when 

 there are no atmospherical perturbations. 3. Ascertain if 

 the connections between the direction of the current and 

 the flow of the rising sea are, as he supposes, inverse, 

 according as the current is regular or irregular. 



NOTES 

 The great osteological collection which Dr. Barnard Davis, 

 F.R.S., has accumulated during a long life devoted to anthro- 

 pological pursuits is well known, both in England and abroad, 

 as the richest and most valuable ever formed by a private indi- 

 vidual, exceeding, as it does, in variety and rarity of the speci- 

 mens all the public collections of this country and most of those 

 on the Continent. It contains about eighteen hundred specimens 

 of skulls and skeletons of various races of men, the value of 



