482 



NATURE 



{March 18, 1880 



stations, both north and south of the line, the difference, if any, 

 is but small. It would appear, however, that both at Batavia 

 and Port Blair, and also at Bombay (judging from the curves 

 civen by Mr. Chambers in his communication previously referred 

 to), that the oscillation when the sun is in southern declination 

 is slightly greater than that pertaining to the summer of the 

 northern hemisphere. At the Russian stations, however, the 

 oscillation of the opposite type is entirely restricted to the winter 

 months, and is therefore far more pronounced in the winter 

 curves in Fig. 2 than in the mean annual curves in Fig. i. 

 Hence it follows that the direct action of the sun on the tropical 

 region is to produce an oscillation such that the pressure is 

 lowest when the sun is most spotted, and it is as a compensation 

 to this action that in the winter season an oscillation of the 

 opposite character is set up on the plains of European and 

 Asiatic Russia; possibly also in the Arctic regions, but this 

 requires verification. Analogy would lead us to anticipate the 

 existence of a similar oscillation in Antarctic latitudes when the 

 sun is in northern declination, but perhaps less concentrated 

 geographically owing to the absence of any dry continental land 

 surface, corresponding to the Siberian and Russian plains. This 

 point mu-t remain for future inquiry. 



While on the whole the Russian curves exhibit the oscillation 

 so distinctly and strongly as to leave no room for doubt as to its 

 reality, they show, nevertheless, that it is liable to great dis- 

 turbances, which at times are so powerful as entirely to neutralise 

 the effect. This will be very apparent if curves be drawn with 

 the original values in the first three columns of the table above 

 given for the winter months ; but the most remarkable instance 

 is that afforded by the winter of the year 1877 (at least of the 

 first two months, for I have not yet received the volume of the 

 Russian Annates for 1878). The mean pressure of December, 

 1877, at stations in Western Siberia, exceeded any on record 

 during the whole period of thirty-one years comprehended in 

 the registers before me ; and it is not a little remarkable that 

 in the [.revious July (the mid-winter of the southern hemisphere) 

 an equally excessive, and (in the eight years for which I have 

 registers) unprecedented pressure characterised South-eastern 

 Australia. These accumulations of pressure were, doubtless, 

 intimately connected with the similar phenomenon which charac- 

 terised the intervening Indo-Malayan region in 1876-78, but the 

 attendant circumstances are as yet by no means fully worked 

 out. 



With respect to the nature of the physical causes which pro- 

 duce that alternating oscillation of pressure between the Indo- 

 Malayan region and the Russian plains, which conforms to the 

 sun-spot cycle, our knowledge is still far too imperfect to all. iw 1 if 

 my attempting any exhaustive analysis. It may, however, be 

 not wholly uninstructive to recapitulate some of the results of 

 recent inquiry which bear upon this point, even admitting, as we 

 must do, that in certain respects they require further verification. 

 Such as they are, they indicate a possible explanation, w Inch I 

 will set forth as briefly as possible. 



Among the best established variations in terrestrial meteoro- 

 logy which conform to the sun-spot cycle, are those of tr 

 cyclones and the general rainfall of the globe, both of which imply 

 a corresponding variation in evaporation and the condensation of 

 vapour. Now the variation of pressure with which we have to deal 

 evidently has its seat in the higher (probably the cloud-forming) 

 strata of the atmosphere. This is not only illustrated in the 

 present instance by the observed relative excess of pressure at 

 the hill stations as compared with the plains, but also follows as 

 a general law from the fact established by Gautier and Koppen, 

 viz., that the temperature of the lowest stratum varies in a 

 manner antagonistic to the observed variation of pressure. It is 

 then a reasonable inference that the principal agency in pro- 

 ducing the observed reduction of pressure at the epoch of sun- 

 spot maximum is the more copious production and ascent of 

 vapour, which may operate in three .different ways. First, by 

 displacing air the density of which is % ths greater ; second, by 

 evolving latent heat in its condensation ; and thirdly, by causing 

 ascending currents, and thus reducing dynamically the pressure 

 of the atmosphere as a whole. The first and second of these 

 processes do not indeed directly reduce the pressure, but only the 

 density of the air stratum, while they increase its volume. In 

 order, therefore, that the observed effect may follow, a portion 

 of the higher atmosphere must be removed, aud this will neces- 

 sarily flow away to regions where the production of vapour is at 

 a minimum, viz., the polar and cooler portion of the temperate 

 zones, and more especially those where a cold, dry land surface 



radiates rapidly under a winter sky. Such an expanse is the 

 great northern plain of European Russia and Western Siberia 

 north of the Altai. H. F. Blanford 



SOCIETIES AND ACADEMIES 

 London 



Royal Society, March 4. — " On the Dynamo-Electric 

 Current and on certain Means to improve its Steadiness." By 

 C. William Siemens, D.C.L., F.R.S. 



The author, after alluding to the early conception by Dr. 

 Werner Siemens, of the dynamo-electric or accumulative principle 

 of generating currents, makes reference to the two papers on the 

 subject presented, the one by Sir Charles Wheatstone and the 

 other by himself, to the Royal Society in February, 1867, The 

 machine then designed by him, and shown in operation on that 

 occasion, is again brought forward with a view of indicating the 



N°1. 



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lAport Commutator 

 336Cart*olu£uni6 2 S^yrnwire. 

 4014 S XTIUs iMjOtux . • 

 51%Cons*oU*li0n&5 5 TI J^fwtre 

 3065 S VJUsijtmuze 

 600 Re^ctLiJix ins pwwuriut* . 





progress that has since taken place in the construction of dynamo- 

 electrical machines, particularly those by Gramme and Siemens 

 von Alteneck. The paper next points out certain drawbacks to 

 the use of these machines, both of them being subject to the 

 disadvantage that an increase of external resistance causes a 

 falling off of the current ; and that, on the other hand, the short 

 circuiting of the outer resistance, through contact between the 

 carbon electrodes of an electric lamp, very much increases the 

 electric excitement of the machine, and the power necessary to 

 maintain its motion, giving rise to rapid heating and destructive 

 sparks in the machine i'.se.t. 



An observation in Sir Charles Wheatstone's paper is referred | 

 to, p anting to the fact that a powerful current is set up in the I 

 shunt circuit of a dynamo-electric machine, which circumstance 

 has -ince been taken advantage of to some extent by Mr. I.add 

 and Mr. Brush, in constructing current generators. 



The principal object of the paper is to establish the conditions 

 under which dynamo-electric machines worked on the shunt 

 principle can be made to give maximum results. A series of 

 tables and diagrams are given, the results of experiments con- 

 ducted by Mr. Lauckert, electrician, employed at the author's 



