448 



NA TURE 



[March 8, jc^oo 



accordance with Newcomb's value obtained from other 

 sources. The great difficulty, by which this distinguished 

 man of science found himself embarrassed in this part of 

 his work (see pp. 158-159 of the treatise quoted above), 

 so much so, indeed, that he well-nigh despaired of 

 arriving at a final conclusion as to the value of tt to be 

 adopted, has now disappeared. The values for the 

 masses of the earth, ;;/(5 + ()=i : 327923, as well as of 

 Venus, w(9)=i :4i499i, as derived from the secular 

 variations, may thus be accepted with confidence.* This 

 is one example showing the theoretical importance of the 

 phenomena here discussed ; possibly the results arrived 

 at may be eventually found to contribute towards 

 removing other difficulties still connected with the theory 

 of planetary motions. 



We are, it seems to me, fairly warranted in assuming 

 the force acting in such a peculiar way on the motion of 

 the terrestrial pole to be identical with that which exerts 

 its influence on the secular variations. As regards the 

 nature and origin of this force, there is a wide field for 

 speculation. A suggestion to which I was led by a dis- 

 cussion on this subject with my colleague, Mr. G. Clark, 

 of this observatory, and which seems worthy perhaps of 

 further investigation, is that the force may stand in some 

 connection with the still very mysterious phenomena of 

 the earth's magnetism. There is certainly one fact which 

 lends some support to this hypothesis, viz. the eccentric 

 position of the earth's magnetic poles. Joule's well- 

 known experiments on magnetic strain in iron bars sug- 

 gest the idea that something similar to the molecular 

 displacement in the iron bar may take place in the body 

 of the earth with regard to its magnetic axis. Such a 

 strain along the axis of maximum magnetic moment 

 would almost necessarily cause a displacement of the axis 

 of figure with regard to the axis of rotation. Only so long 

 as the total magnetic potential of the earth was not 

 subject to alterations could this displacement remain 

 constant. In that case the pole of rotation would 

 describe a circle with a constant radius round the pole 

 of figure. But there are facts which force us to assume 

 that the potency of the earth's magnetic forces varies 

 with the state of solar activity, and that consequently 

 the molecular displacement in 'the direction of the mag- 

 netic axis varies accordingly. The most striking fact in 

 this respect is the increase of auroras with an increasing 

 number of solar spots. Now, if we were to consider 

 aurorje as discharges of electric force gradually accu- 

 mulated in the earth's interior, the strain in the direction 

 of the magneti* axis should have abated after such a 

 discharge, ancf the pole of figure should therefore 

 approach the pole of instantaneous rotation. This, then, 

 would explain the fact that the semi-amplitude of latitude- 

 variation is smallest after a maximum display of solar 

 spots. How far this hypothesis is able to account for 

 other phenomena brought out by observation must be 

 left to future research. J. Halm. 



Note. — In a very interesting note in Monthly Notices, March 

 1 898, Mr. Thackeray investigates the effect of the latitude-varia- 

 tion on the longitudes of the sun as well as on the arc of the 

 obliquity. It appears from his result that the correction to the 

 .sun's longitude due to the motion of the pole has an amplitude 

 two and a half times greater than that to the obliquity, cosec. e 

 heing almost exactly 2'5. Now from Prof. Newcomb's and Mr. 

 Thackeray's tables showing the corrections to the right ascen- 

 sions of the sun relative to the stars, as derived from Greenwich 

 observations, I have computed the numerical effect of an error 

 produced in the sun's longitude by an increase from o to 10 in 

 Wolfs relative spot-numbers. Assuming Prof. Newcomb's 

 weights assigned to the observations, I obtained AA=o"-4457+: 

 c"-o523, and consequently AA sin « = o"'i775 ± o"'0209. On 

 the other hand, the value previously found for the obliquity in 

 Astr. Nach. 3619, was Ae = o"'i703 ± o""oi45. Hence the 



1 This value of the mass of Venus is in close accordance with that 

 derived by Prof. Backlund from the perturbations of Venus on Encke's 

 Comet. 



NO. 1584, VOL. 61] 



result that the effect of the force varjing with the great spot- 

 period is in point of anipiilude exactly analogous to that '^cnnd 

 by Mr. Thackeray with regard to the phenomenon of latitude- 

 variation. This must, I think, point to the conclusion that thvise 

 anomalies in the sun's longitude and in ihe arc of the obliquity 

 which correspond to the great sunspot-period are due to the 

 same force which causes the deviations observed in the motion 

 of the terrestrial pole, that in fact these anomalies are merely 

 another proof of our assertion that the displacement of the pule 

 stands in some connection wiih the variation of solar activity. 

 The results are derived from two perfectly independent series of 

 observations, the probable errors of AA and As being not more 

 than one-tenth of the values obtained ; hence the assumption as 

 to the peculiarities elicited from the observations of the sun's 

 right-ascensions and declinations being the result of chance 

 appears to be absolutely untenable. J. H. 



A FPU ED ME TEOROLOG K* 

 T^HE name of Prof. Cleveland Abbe is indelibly 

 -*■ associated with the onward progress of meteorology 

 in the United States, whether in respect of its com- 

 mercial and general utility, or on the severer but not less 

 interesting side of dynamical investigation, to which we 

 must look for any theoretical advancement. In the 

 organisation of an immense service, the observations and 

 returns of which cover a large field in the applied science, 

 he has played a yeoman's part ; and further, by his trans- 

 lations of many important mathematical papers, he has 

 stimulated study, and given to his countrymen the oppor- 

 tunity of familiarising themselves with the difficult theory 

 which Helmholtz, Hertz, Kirchhoff and others have 

 developed, and on which the perfected science must 

 eventually be based. It seems fitting, therefore, when 

 one of the States — in this case, Maryland — proposes to 

 advance beyond the mere accumulation of meteorological 

 data, and is prepared to foster the study and discussion 

 of special problems connected with climate and its 

 effects, to devote funds for the prosecution of certain 

 lines of research and investigation, that he should be 

 selected to indicate the direction in which study can be 

 most profitably turned with the view of benefiting every 

 human interest. We at least may congratulate ourselves 

 upon the selection, for the outcome has been to collect 

 into an essay of moderate dimensions a list of the most 

 prominent fields of usefulness to which a State weather 

 service can devote itself. We have here a scheine, which 

 though in the first place intended for a particular climate 

 and district, yet is not limited in its scope, and but with 

 {&\s changes may be made to serve as a model for wider 

 areas, and in its fullest application would meet the 

 demands and the necessities of the best instructed 

 meteorologists. 



It would be wearisome to give a bare statement of the 

 manifold duties and occupations in which a properly 

 constituted weather service finds itself called upon to 

 take part, and in these pages, fortunately, such a task is 

 not necessary. There may be some who think that 

 weather prediction for shorter or longer periods alone 

 occupies the attention and satisfies the ambition of the 

 meteorologist. To such we commend the list of twenty- 

 nine distinct subjects of enquiry, bearing on every walk 

 of life, assisting every one of the applied sciences, and 

 touching every material interest, that Prof Cleveland 

 Abbe has put in the foreground of his essay, as additions 

 to the daily storm and weather forecasts, admitted by the 

 least attentive to be the main duty of a meteorological 

 office. But it is needless to say that the author is not 

 satisfied with the mere utilitarian aspect of the science, 

 however valuable the results may be to the agriculturist, 

 the engineer, the mariner, the physician and others 

 throughout the entire circle of the professions. Just as 



1 "The Aims and Methods of Meteorological Work, especially as con- 

 ducted by National and State Weather Services." By Cleveland Abbe. 

 (Baltimore: Johns Hopkins Press, 1899.) 



"The Monsoon Forecast." Tke Pio?ieer, August 10 and October 23, 181^9. 



