Inquiry into the Origin of Mud Bushes in De Beefs Mine. 61 



Now, given a sufficient quantity of mud in the vicinity of the 

 underground workings, what possible influences, over and above its 

 own weight, could be conceived as likely to set it in motion ? I can 

 only think of three : — ■ 



1. Landslips due to undermining ; 



2. Earth tremors ; 



3. Changes of atmospheric pressure. 



The first is not a matter that admits of scientific discussion. In 

 the whole of South Africa there are, unfortunately, no instruments 

 which would have enabled one to deal with the second ; we are 

 therefore restricted to the third. 



In order to consider the matter as judicially as possible, I have 

 thought it best to plot the barometer curve, from, observations taken 

 at 8 a.m., 2 p.m., and 8 p.m., daily, for the whole thirty-six months, 

 irrespective of whether a mud rush occurred in any assigned month 

 or not. The curve is a very open one — twenty inches of paper to 

 one of mercury* — with the idea of accentuating the differences of 

 pressure. On the day, and as near the time as could be ascertained, 

 that a mud rush took place, m or M is written under the curve, a 

 small letter or a capital being used according as the account seemed 

 to warrant a small or large rush respectively. W indicates a rush of 

 water. The arrow-heads attached to most of the letters indicate 

 roughly from which end of the working area the mud came. The 

 attached numbers show the depth of the level upon which the mud 

 started. The vertical lines at the base of the diagram represent the 

 rainfall — an inch of paper to an inch of rain. The horizontal lines 

 through the barometer curve show the mean pressure of the month. 

 The dots indicate the maximum shade temperatures of the day. It 

 is important to remember that the mean of the pressures at 8 a.m., 

 2 p.m., and 8 p.m., is almost identical with the mean of the hourly 

 values, and also that in general the maximum and minimum pressures 

 will be higher and lower respectively than the crests and depressions 

 of the curve. 



A study of these curves leaves, I think, no room for doubt that 

 changes of atmospheric pressure are the most potent of all ex- 

 traneous causes tending to drive out the mud. Only a few mud 

 rushes take place under a steady barometer, nearly all while the 

 changes of pressure are rapid and the gradients steep. The greater 

 number occur during the passage of a depression, and herein, though 

 the analogy might be unduly pressed, remind one of the meteoro- 

 logical characteristics of choke-damp. Of the whole fifty-two mud 

 rushes, twenty-five took place when the barometer was below the 

 * These dimensions have been considerably reduced in printing. 



