Tidal Phenomena at Inland Boreholes near Cradock. 
97 
Similarly, the co-ordinates of all the turning-points for that fortnight 
were determined on the tide gauge records of the South African ports of 
Cape Town, Port Elizabeth, East London, and Durban. 
For general comparison all these measured data were plotted in 
parallel lines on the same time scale, and the general resemblance of 
the well curve to the curves of the coastal tide records demonstrated. 
The original Tarka Bridge record for the fortnight was then subjected 
to a process of harmonic analysis for the purpose of determining the 
periods of the principal harmonic components of the curve. The 
particular method used was described by Chrystal as the method of 
" Eesiduation " (Trans. Roy. Soc, Edinburgh, vol. xlv., part 2, pp. 385-7). 
This method is applicable to comparatively short curves and involves 
no assumptions as to periods or the causes operating to produce 
the curve. One by one the various simple harmonic components are 
disentangled from the compound curve with their periods unaltered 
but with their amplitudes considerably reduced. 
In this way components were isolated from the Tarka Bridge curve 
having the following wave periods : — 
1. 12 hrs. 27 min. [probably divisible into 12 hrs. 26 min. and 
12 hrs. 0 min.] 
2. 23 hrs. 57 min. 
3. An anharmonic residuum which appeared to be the vertical inversion 
of the barograph curve for the fortnight. 
Component No. 3 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 con- 
ducted 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 hrs. 25 min. 14f sec. 
2. Principal Solar semi-diurnal tide — period 12 hrs. 
3. Three diurnal tides with periods — 23 hrs. 56 min. ; 24 hrs. 4 min, ; 
25 hrs. 40 min. 9J sec. 
General Bemarks. 
The foregoing results seem to establish beyond question that the 
fluctuations in these wells are to be attributed directly or indirectly to 
extra-terrestrial causes, but the precise nature of the connection is not 
by any means clear. 
7 
