64 Transactions of the South African Philosophical Society. 



the air, a lot of the mud will instantly burst out of the mouth of 

 the pot. 



Suppose now a given mass of mine debris to be saturated, and to 

 be held in equilibrium by the atmosphere at mean pressure. The 

 effect of an increase of pressure would be simply to check the per- 

 colation of water from the rock. On the other hand, a decrease of 

 pressure by diminishing the conservative forces, would tend to set 

 free the mud. The barometer curves do, I think, support this 

 view. 



That every depression is not accompanied by a rush of mud may 

 be due either to a lack of mud for the time being, or to something 

 blocking the exit, or to the fact that, in a well-ventilated mine, with 

 upcast and downcast shafts, a surface barometric depression may not 

 necessarily indicate a decrease of pressure in the working places. 

 Let us consider how the surface temperatures might be expected 

 to act. Though the data are not yet sufficient for a rigid mathe- 

 matical analysis, their general tendency will be, perhaps, obvious 

 enough. So long as the surface air is warmer than the air in the 

 working places, there will be, as a rule, equilibrium in the shafts 

 and tunnels ; but immediately the temperature of the surface air 

 falls below that of the working places the equilibrium will be dis- 

 turbed. The warm air from below will rise in the upcast shaft, and 

 the colder surface air will flow down the downcast shaft (in the 

 present instance the Eock Shaft) to take its place. As a consequence 

 a current of air will be created, its strength varying in proportion to 

 the difference of temperatures. The downcast current of cold air 

 will be warmed as it descends, chiefly by conduction from the heated 

 chambers, and hence it will not in general penetrate directly into the 

 offsets and working places. On the contrary it will flow rapidly past 

 their entrances along such tunnels as permit of its free ingress and 

 egress. The important point to be borne in mind, however, is that 

 the flow of the current past the offsets and working places will lower 

 the pressures within them by suction — an operation the principle of 

 which has been utilised in the anemometers of Dines and Hage- 

 mann.* There are not any data upon which one could base even a 

 guess as to the amount of diminution of pressure such a process 

 could be expected to produce. At some future time I hope to be 

 able to take systematic observations of the air currents in the 

 Kimberley Mines, and this part of the question may then, it may be, 

 receive some sort of solution. Meanwhile it seems clear that the 

 general tendency of the temperatures is to modify the pressures : 



* These anemometers, however, project into the air ; the months of the working- 

 places are more often than not flush with the tunnels. 



