Sept. 25, 1879] 



NATURE 



50- 



numerous scientific journals of to-day. We look forward 

 to the time when the Annual Record of Science will 

 become a standaid work absolutely indispensable to all 

 libraries, both public and private, at home and abroad. 



W. F. B. 



LETTERS TO THE EDITOR 



[ The Editor does not hold himself responsible for opinions expressed, 

 by his correspondents. Neither can he undertake to return, or 

 to correspond with the writers of, rejected manuscripts. No 

 notice is taken of anonymous communications. 



[ The Editor urgently requests correspondents to keep their letters as 

 short as possible. 7 he pressure on his space is so great that it 

 is impossible otherwise to ensure the appearance even of com- 

 munications containing interesting and novel /acts.'\ 



Heat of the Comstock Mine 



I NOTICE in Nature, vol. xx. p. 168, that Dr. Lesley quotes 

 from Prof. Barker an opinion in regard to the heat of the Com- 

 stock Mines in Nevada. Referring to my assertion that the heat 

 of the rock "is pretty uniform" in the lower levels, Prof. Barker 

 announces that there are " the most remarkable differences, some 

 of the higher levels being much hotter than some of the lower 

 levels." This is perfectly true, and the fact is no disproof of 

 my assertion. In the article to which Dr. Lesley refers (Silli- 

 man^s Journal, April, 1879) I said that there are striking differ- 

 ences of temperature in the rock, and endeavoured to explain 

 them by showing that there is a great mass of rock which may 

 be regarded as heated to a tolerably uniform degree at all points 

 in the length of the hide, on any given level, and that in this 

 general mass there are isolated localities, most of which show a 

 temperature above that of the rock at large, but some of them 

 below it. I pointed out the conditions under which these local 

 maxima occur, and gave the explanation to which I thought 

 they led. The hot spots are evidently narrow and long, and as 

 the mine openings sometimes intersect and sometimes follow 

 them for some distance, a given level will be for a part of its 

 length in a hot belt and for a part in the general mass of heated 

 rock, or one level may be in a hot belt and show a much higher 

 temperature than the level below, which entirely escapes the 

 exceptionally hot ground. In this way thermometric variations 

 are obtained between different levels and between different parts 

 of the same levels, and these facts were all brought out in 

 my article. 



I should not trouble you with this explanation did I not feel 

 that the Comstoik lode bids fair to become a classic field for the 

 discussion of terrestrial temperatures. Mr. Clarence King is now 

 on the ground, and will, no doubt, make its unrivalled heat 

 phenomena the subject of careful examination, and everything 

 that bears upon the question has importance. 



Dr. Lesley expresses some doubt upon the mechanical theory 

 of earth-heat which was one of Prof. Barker's two conclusions 

 upon the source of the heat. The Comstock is certainly good 

 ground to test this question, for I have never witnessed sucli 

 constant and general movement of the rocks in any other mines. 

 Still, I do not share Prof. Barker's opinion on this, or on his 

 other point, "that the heat is a hot-water heat." No mining 

 engineer would pronounce the Comstock a wet lode. It dis- 

 charges four and a half million tons of water yearly, and yet out 

 of the more than twelve miles of linear excavation made every 

 year, I do not believe that 1,000 feet are in ordinarily wet ground. 

 It is a dry lode for the greater part, and in writing upon the 

 subject my efforts have been directed to seeking an explanation 

 for the extraordinary temperature of this dry rock. 



John A. Church 



IIS, Broadway, New York, September 8 



paper offices and four meeting-rooms — were teleop-aphically con- 

 nected with the Cutlers' Hail, where a switch-board stood to 

 place any two distant stations into communication, thus illustrat. 



Crossley's Modification of Hughes's Microphone 



Ever since Hughes's discovery of those principles which led 

 to his invention of the microphone, inventors have been trying 

 to improve the instrument by adopting every variety of form and 

 employing every combination of apparatus that were likely to 

 lead to good results. The failures must have been legion, and 

 of the successes the members of the British Association have 

 had during their s'ay at Sheffield, an opportunity of examining 

 and feeing at work ptrhaps the most efficient — Crossley's modi- 

 ficttion of the microphone. Six distant places — the two news- 



ing the exchange system so largely employed in America. Every 

 one is aware that with the telephone the speaker has to hold the 



instrument to his mouth ; with the Crossley's trarsmitter, how- 

 ever, conversation, a few feet nway, is readily conveyed. The 

 transmitter is now being Lr^tly imployed ni lh«: United King- 



