1873O Limits of our Coal Supply, 353 



such conditions, yet such persons are to be found. The 

 work of stoking or feeding the fires is usually done by Arabs, 

 while the work of bringing the coal from the bunkers 

 is done by sidi-wallahs or negroes. At times some of the 

 more intelligent of these are promoted to the stoking. The 

 negroes who do this kind of work come from Zanzibar. 

 They are generally short men, with strong limbs, round 

 bullet heads, and the very best of good nature in their dis- 

 positions. Some of them will work half an hour in such a 

 place as the stoke-hole without a drop of perspiration 

 on their dark skins. Others, particularly the Arabs, when 

 it is so hot as it often is in the Red Sea have to be carried 

 up in a fainting condition, and are restored to animation by 

 dashing buckets of water over them as they lie on deck." 



It must be remembered that the theoretical temperature 

 of 116 at 4000 feet, the 133 at 5000 feet, or the 150 at 

 6000 feet, are the temperatures of the undisturbed rock ; 

 that this rock is a bad conductor of heat, whose surface 

 may be considerably cooled by radiation and convection ; 

 and therefore we are by no means to regard the rock tempe- 

 rature as that of the air of the roads and workings of the 

 deep coal pits of the future. It is true that the Royal Com- 

 missioners have collected many facts showing that the 

 actual difference between the face of the rocks. of certain 

 pits and the air passing through them is but small ; 

 but these data are not directly applicable to the question 

 under consideration for the three following reasons : — 



First. The comparisons are made between the tempera- 

 ture of the air and the actual temperature of the opened and 

 already-cooled strata, while the question to be solved is the 

 difference between the theoretical temperature of the un- 

 opened earth depths and that of the air in roads and work- 

 ings to be opened through them. 



Second. The cooling effect of ventilation must (as the 

 Commissioners themselves state) increase in a ratio which 

 " somewhat exceeds the ratio of the difference between the 

 temperature of the air and that of the surrounding surface 

 with which it is in contact." Thus, the lower we proceed 

 the more and more effectively cooling must a given amount 

 of ventilation become. 



The third, and by far the most important, reason is, that 

 in the deep mining of the future, special means will be 

 devised and applied to the purpose of lowering the tempera- 

 ture of the workings, that as the descending efforts of the 

 collier increase with ascending value of the coal, a new 

 problem will be offered for solution, and the method of 



