96 Transactions of the Boyal Society of South Africa. 
of approaching storms in this way. It seems probable that great falls in 
the barometric pressure would increase the rate of gas delivery, by 
allowing the expansion and partial escape of quantities of gas partially 
imprisoned in underground branch fissures with low outlets communicating 
with the main fissures forming the water pathways. 
Summary. 
Observations begun in 1905 and carried on at intervals until the 
present year on a group of wells on a farm at Tarka Bridge, Cradock 
District, are described in detail. 
The wells have not been bored very deep, the deepest being 225 feet, 
but it is obvious that the bores connect with deeply extending fissures, as 
the waters issue at temperatures of about 80° accompanied by large 
quantities of natural inflammable gas (methane), while sulphuretted 
hydrogen is present in notable quantities in solution in the water. The 
wells are 2,700 feet above sea-level and over 100 miles from the coast. 
Measurements of the pressure at which the water issues show a 
remarkable fluctuation, in some respects analogous to the tidal fluc- 
tuations of the sea. 
A series of direct measurements covering several days established the 
fact that there was a real fluctuation both in the amount of water 
discharged and in the well-pressure. Continuous records were then 
obtained over longer periods by means of clock-driven, self-recording 
apparatus in order to study the precise nature of the fluctuations. 
The longest continuous record obtained extends over a period of 
fifteen weeks. This graphical record shows that the semi-diurnal fluctua- 
tions attain a maximum amplitude at fortnightly intervals at times cor- 
responding to the times of new moon and full moon throughout the 
fifteen- weeks' period. 
This record further demonstrates the fact that the mean daily water 
pressure rises with each fall of barometric pressure, and falls with each 
rise in barometric pressure as recorded concurrently at the farm by means 
of a barograph instrument. The time scale on this fifteen-week record is 
about 11 inches per week. 
Eecords obtained for shorter periods on a time scale of 13^ inches per 
day were found to be much more suited for detailed critical examination 
and analysis. In particular, the record for a certain fortnight during 
which the barometric pressure was very steady (and consequently its 
interfering effect almost negligible) was selected. The times of all the 
turning-points were carefully determined in terms of South African 
official time. The heights of all the turning-points of the curve were 
also determined in inches. 
