392 SCIENCE PROGRESS 



calculated degree of saturation of different parts of the body 

 during a dive to 6 atmospheres pressure for fourteen minutes 

 with the old and the new method. It will be seen at once 

 that the slow descent adds greatly to the danger, while during 

 most of the uniform ascent many parts of the body are actually 

 increasing in saturation ; so that by the old method a diver 

 runs a very serious risk of death or paralysis when he reaches 

 surface. By the new method this risk is avoided, and the total 

 time under water is reduced to nearly half. To obtain equally 

 safe results by the method of uniform decompression it would, 

 to judge from the experiments on goats, be necessary to 

 extend the duration of decompression to several hours, the 

 reason being that, in the case of short exposures to high pres- 

 sure, not only is time wasted during the first half, or two-thirds, 

 of the uniform decompression, but the saturation of most parts 

 of the body continues to increase very seriously, so that at the 

 end the decompression rate is far too fast unless the process 

 is made very slow indeed. 



After long exposures to compressed air, as in work in 

 caissons and tunnels at moderate pressures, the saving in time 

 by stage decompression is not so striking ; but very substantial 

 gains in convenience become possible. Time, however, forbids 

 me from entering further into this subject, and I must now 

 return to work in mines. 



One of the most formidable difficulties met with in the deep 

 mining which is now becoming so common is the great heat 

 which is encountered. It is well known that the deeper we 

 penetrate the earth's crust the higher is the natural temperature 

 of the rock. The rate of increase is usually about 2 , 5° C. per 

 ioo metres. At some places it is more than this : at others 

 less. At Johannesburg, for instance, the increase is only o'9° 

 per ioo metres, while in Cornwall it is 3 . At a depth of 

 1,000 metres, which is now becoming common, the natural 

 rock temperature is usually about equal to the body tempera- 

 ture. The air temperature in a mine may be either above or 

 below the natural rock temperature. Fresh air direct from 

 surface, or the evaporation of moisture, may cause a local 

 reduction in mine temperature. On the other hand the slow 

 oxidation of mineral substances, such as iron pyrites (FeS 2 ), 

 very commonly causes the mine temperature to rise consider- 

 ably above the original rock temperature. Miners are thus 



