72 Transactions of the Boyal Society of South Africa. 
On inquiry I find that Peg. 91 is 2,706 feet above railway level sea datum. 
The borehole levels then become — 
No. II. 2,707 feet above sea-level 
No. III. 2,697 
No. IV. 2,710 
No. V. 2,713-7 
It has been already mentioned that the water of No. V. when piped 
upwards rose about a foot above the datum mark. This indicates an 
upward pressure of the water at the top of the borehole of less than -|- lb. 
on the square inch. 
In the case of No. III. the pressure was much greater, and by closing 
that hole and balancing its pressure in a U-tube against a column of 
mercury I found that at first the pressure was equivalent to 10 inches of 
mercury column, rising during the first 5 minutes to 12*1 inches, and after 
10 hours to 14*1 inches of mercury — a pressure of about 7 lbs. on the 
square inch. In other words, while the potential level of No. V. water 
was less than a foot above the borehole orifice, the potential level of 
No. III. was about 16 feet above the orifice. Incidentally we may also 
remark that the potential levels of No. V. and No. III. as thus determined 
were nearly the same, namely, 2,714^ and 2,713 feet above sea-level. 
The foregoing data of relative levels and pressures seem to suggest a 
sufficiently plausible explanation for the comparative absence of very 
notable fluctuations in the yields of Nos. II. and III. The water of 
No. V. issues at so small a pressure that an increment of pressure 
amounting to an ounce or less on the square inch would naturally produce 
a noticeable effect, while a similarly small increment added to an already 
existing pressure of 6 or 7 lbs. on the square inch at No. III. might affect 
the yield there by an amount which might easily escape observation. 
We also have the suggestion that if No. III. were piped up to the 
neighbourhood of its potential level it might be possible to detect fluctua- 
tions similar to those of No. V. Later it will be seen that this suggestion 
was carried out with results that confirmed the general truth of the above 
tentative reasoning. 
On my return to Cape Town after my first visit to Tarka Bridge I had 
a self-recording instrument made there with such materials as I could 
find. Mr. Brackenbury Baily, of the Telegraph Department, kindly lent 
me a 4^-inch diameter revolving drum, with a fairly good spring-driven 
clock driving it through one revolution a day. Mr. W. H. Cottell, a highly 
skilled fine mechanic with the training of a chronometer maker, was at 
that time in the employment of the Public Works Department, and he 
carried out the construction of the machine under my direction. 
