590 



NATURE 



W^ 



[October 22, 1891 



polates in others crude statements which render them 

 ridiculous, he does an injustice to the authors to whom he 

 acknowledges his indebtedness, and he shirks responsi- 

 bility by saying that " these notes do not lay claim to 

 originality." Could anything be more misleading than 

 the following description of sun-spots on p. 148 ? " They 

 seem to rise suddenly to a great height, cool, and then 

 sink back into the photosphere. They are due to up- 

 rushes of incandescent hydrogen, and are identical with 

 the red flames seen during an eclipse." And the figure 

 that accompanies this text cannot be a sun-spot at all, 

 but must be something else inserted by mistake. Another 

 blunder occurs on p. 59, where a section of an intermittent 

 spring is shown upside down. The figures are mostly very 

 coarse and poor, especially the moraines on p. 62, the 

 section through a cinder cone on p. 89, and one of a 

 volcano on p. 90 ; whilst the two figures of ocean bottoms 

 on pp. 102 and 103 give a very wrong idea of their nature. 

 There is, of course, a deal of information in the book, but 

 no attempt is made to give it interest. In fact, although 

 the author is a teacher of physiography, it is very evident 

 from his work that he has not paid attention to the 

 practical side of his science, or verified any of the pheno- 

 mena he essays to describe. As a book of reference the 

 work before us is untrustworthy ; and as a work for 

 students of elementary physiography it is useless and 

 much to be condemned. 



Thomas Sopzvtth, M.A., C.E., F.A'.S.j with Excerpts 

 from his Diary of Fifty-seven Years. By B. Ward 

 Richardson, F.R.S. (London: Longmans, Green, and 

 Co., 1 89 1.) 

 Mr. Sopwith died in 1879 ^^ the age of seventy-six. He 

 was not eminent as an original scientific investigator, but 

 he was a man of great vigour and freshness of mind, 

 and had won the affection of a wide circle of friends 

 by his genial and happy temper. For many years he 

 resided at Newcastle as an engineer and railway surveyor. 

 Afterwards he removed to Allenheads, where he served 

 as the chief agent of Mr. T. \V. Beaumont's lead-mines 

 in Northumberland and Durham. Dr. Richardson's 

 book will recall Mr. Sopwith vividly to the minds of his 

 friends, and it contains many things which will be of 

 interest even to readers who were not personally ac- 

 quainted with him. During the long period of fifty-seven 

 years he kept a diary regularly ; and of this, of course. 

 Dr. Richardson has made liberal use. The extracts 

 show that Mr. Sopwith studied closely the currents of 

 scientific opinion, and formed his own judgment about 

 them in a shrewd and independent spirit. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. NtWier can he undertake 

 to return, or to correspond with the 2uriters of, rejected 

 manuscripts intended for this or any other part of N^TVKV.. 

 No notice is taken of anonymous communications. "X 



Electric Transmission of Power. 



Your article of the ist inst. on the International Electrical Ex- 

 hibition (p. 522), says : " In those days (before 1879) two wrong 

 notions misled people — the one, that the maximum efficiency of 

 a perfect electromotor could be only 50 per cent. ; the other, 

 quoting the remarks of Sir W. Siemens, 'in order to get the 

 best effect out of a dynamo-electric machine, there should be an 

 external resistance not exceeding the resistance of the wire in the 

 machine.'" 



These two notions are really one : the first follows by 

 immediate inference from the second. 



Your article says a little further on : "At the British Associa- 

 tion in 1879, Prof. Ayrfon exposed the fallacy of assuming that 

 50 per cent, was the maximum efficiency theoretically obtain- 

 able from an electromotor. . . . This was perhaps the first time 



NO. II 47, VOL. 44] 



that it had ever been suggested that the efficiency in electric 

 transmission of power could be more than 50 percent." 



This is a mistake as to historical fact. Many years ago, I am 

 not sure of the date, but it was long before the dynamo was 

 invented, I had some conversation with the late Prof. Joule 

 about mechanical equivalents and motive power, in which he 

 told me that an electromotor (worked, of course, by a voltaic 

 battery) had shown a very high percentage of efficiency— I think 

 he said 79 per cent., and I am sure it was far above 50. I said, 

 " How is that compatible with Ohm's demonstration that the 

 efficiency of an electric circuit is at a. maximum when the re- 

 sistance of the battery is equal to that of the rest of the circuit ? " 

 tp which he replied, "The maximum effect, in Ohm's theorem, 

 does not mean the maximum work done by the oxidation of a 

 "given quantity of zinc, but the maximum effect obtainable from 

 a given surface of zinc plates." " I see," said I, "just as in 

 ■the case of the steam-engine, the problem of getting the maxi- 

 mum of useful effect from a given weight of coals is a different 

 one from that of getting the maximum of power from a given 

 area of piston." 



This appears to be an instance of a truth being grasped by 

 one of the great masters of science long before it passed into 

 general teaching. And it is also an instance of a truth being so 

 mistaken as to mislead : Ohm's law was evidently understood 

 to bear a significance that it did not really bear. 



Belfast, October 13. Joseph John Murphy. 



[That Joule had clear and correct views regarding the efficiency 

 of an electromotor driven by a voltaic battery was pointed out 

 some years ago, being mentioned, for example, by Prof. S. P. 

 Thompson in his book on " Dynamo- Electric Machinery." But 

 in the paragraph quoted by Mr. Murphy from Nature of 

 October i, the expression "electric transmission of power" had 

 reference to the combination of apparatus exhibited at the 

 lecture in question — had, in fact, the meaning usually attached 

 to this expression, viz. the employment of a dynamo to convert 

 mechanical energy into electric energy at one end of a pair of 

 wires of some length, and the employment of a second dynamo 

 at the other end of the wires to convert the electric energy back 

 again into mechanical energy. 



Now, not only would it have been somewhat difficult to foretell 

 what would be the combined efficiency attainable by the employ- 

 ment of two dynamos as generator and motor, at a period " long 

 before the dynamo was invented," but even down to 1879 no 

 one had succeeded in practically transmitting power by means of 

 this combination with an efficiency of as much as 50 per cent, 

 over a distance of even one mile. 



The only direct-current dynamo in common use at that date 

 was the series dynamo, and that machine, as is well known, 

 differs radically in its behaviour from a voltaic battery. For 

 while it is when a voltaic battery is developing a very small 

 current that it gives power most economically to the 

 outside circuit, the series-dynamo, when only a very small 

 current is passing through it, develops practically no electro- 

 motive force, no power, and therefore has a very low efficiency. 

 Hence, although electricians were undoubtedly mistaken in 

 fancying that there was a theoretical limit of 50 per cent, in the 

 efficiency when two dynamos were employed in the transmission 

 of power, neither the error, nor its correction, were of that 

 obvious character in 1879 that one might imagine from reading 

 Mr, Murphy's letter.— -W. E. A.] 



Rain-making. 



In 1883 I published in Nature (vol. xxviii., p. 83) an 

 account of some experiments which I made to explain the 

 curious phenomenon commonly seen at the Bocca of the Solfatara 

 of Pozzuoli : paper or brushwood is kindled near the fumarole, 

 and the action of the flame, even when its duration has been 

 very brief, is observed for some time after in the relatively great 

 increase of cloudy vapour that appears to roll out of the Bocca 

 and to rise from the surrounding minor fumaroles. According 

 to Prof. Arcangelo Scacchi, this increasd condensation of 

 vapour is due to the carbon dioxide produced in the combustion ; 

 this gas causing condensation from the highly saturated medium 

 in the same way as fumes become visible when concentrated 

 hydrochloric acid is exposed to ordinary air. My experiments 

 of 1883 tend to show that not only carbon dioxide, but (in ac- 

 cordance with the views of Dr. Aitken on the formation of 

 cloud or mist) the increase of solid corpuscles made to float in 



