Minutes of Proceedings. 
xxiii 
In this way components were isolated from the Tarka Bridge curve, 
having the following wave periods : — 
1. 12 hours 25 minutes. 
2. 12 hours 0 minutes. 
3. 23 hours 75 minutes. 
4. An unharmonic residuum which proved to be the vertical inversion 
of the barograph curve for the fortnight. 
Component No. 13 was obviously not a simple harmonic function. It 
was apparently composed of several harmonics, of approximately diurnal 
period, but on the scale on which the analysis was being conducted the 
practical limit of the method had been reached. Accordingly no finer 
dissection was attempted. 
The above results may be compared with the well-known principal 
harmonic components of Marine Tides. 
1. Principal Lunar semi-diurnal Tide : Period 12 hours 25 minutes 
14 l-6th seconds. 
2. Principal Solar semi-diurnal Tide : Period 12 hours. 
3. Three diurnal tides and periods : — 
23 hours 56 minutes. 
24 hours 4 minutes. 
25 hours 40 minutes 9J seconds. 
The foregoing results seem to establish beyond question that the 
fluctuations in these wells are to be attributed directly or indirectly to 
astronomical causes, but the precise nature of the connection is not by 
any means clear. 
The wells are situated over 100 miles from the coast, at an altitude of 
over 2,700 feet above sea-level. High water at Tarka Bridge occurs about 
14|- hours after high water at East London, while the lag in the case of 
low water is nearly 15 hours. 
The principal conceivable theories to account for the phenomena would 
appear to group themselves in three classes : — 
{a) Theories depending on the direct gravitative influence of the sun 
and moon on the land or the underground water. 
ih) Theories depending on the action of the Marine Tides on the coast 
loading and distorting the land, 
(c) Theories depending on the action of Marine Tides in periodically 
reducing the freedom of outflow of underground water through 
submarine springs. 
No attempt is at present made to state or discuss these theories. It is 
felt that a satisfactory theory can be arrived at only by the co-operative 
discussion of the subject by astonomers, geologists, and hydraulicians. 
Tidal wells are known in many parts of the world, but practically all are 
within three or four miles of the seashore, and at no considerable altitude. 
5. 
