438 Further Note on the Coal Supply. 



a variety of considerations ; but where the maize is extensively 

 grown, and there are local facilities for manufacture and transport, 

 it would seem well worth the consideration of practical men. As 

 the seed of the plant sells for sufficient to give a profit on the cost 

 of cultivation in suitable localities, and as three useful sub- 

 stances, amounting in the aggregate to 130 lbs. out of three 

 hundred weight of leaves, can be extracted, there is ground to 

 hope that the process may pay. The pecuniary consideration is, 

 however, foreign to our objects, and if we had fully studied 

 them, we should decline offering an opinion. All that belongs 

 to us to say, is, that Dr. Welsbach has displayed considerable 

 technological skill in producing the series of objects that have 

 been sent for our examination, and we hope his labours will be 

 of value in creating a new branch of industry, and augmenting 

 the supply of materials in daily and increasing demand. 



FURTHER NOTE ON THE COAL SUPPLY. 



BY PROFESSOR ANSTED, F.R.S. 



My attention has been drawn to an error more unaccountable 

 than really important, in reference to the argument in my 

 article on the " Supply and Waste of Coal," in your last number. 

 I have said, in page 322, that an acre of land contains 2840 

 square yards. I need not remark that the real figure is 4840 ; 

 but what was originally, probably, a slip of the pen, I have 

 inadvertently carried out in a little calculation with reference to 

 the quantity of coal in each square mile, one foot thick ; and 

 again in the estimated grand total of coal under certain proba- 

 bilities (p. 323), and the available remainder, in page 325. I 

 can no more account for the error than in the case of a mistaken 

 date ; but the reader may observe that the value of the argu- 

 ment is really unaffected. I had no intention to do more than 

 give a general illustration, and it matters little whether the 

 actual quantity of coal left is estimated at thirty- five or sixty-five 

 thousands of millions of tons, the present consumption being a 

 hundred millions, and rapidly increasing. 



The figures should be as follows : — 



Each square mile of coal, one foot thick, contains 968,000 

 tons, and if the mean thickness is fifty feet, and the area 

 6000 square miles, the quantity is 290,400 millions of tons. 

 The fourth part of this available is 72,600 millions, and the 

 remainder 65,000 millions. This would be exhausted in a few 

 under the supposed conditions, and economy is there- 

 fore loudly called for. 



I must apologise to your readers for an error that certainly 

 ought not to have been allowed,but of which Icangiveno account. 



