Afar. 28, 1872] 



NATURE 



423 



FURTHER INVESTIGA TIONS ONPLANETAR Y 

 INFLUENCE UPON SOLAR ACTIVITY* 



I. T N a previous communication by us to this Society, 

 -'■ an abstract of which was pubhshed in the Pro- 

 ceedings, vol. xiv. p. 59,f we showed some grounds for 

 beheving that the behaviour of sun-spots with regard to 

 increase and diminution, as they pass across the sun's 

 visible disc, is not altogether of an arbitrary nature. 

 From the information which we then had, we were led to 

 think that during a period of several months sun-spots 

 will, on the whole, attain their minimum of size at the 

 centre of the disc. They will then alter their behaviour 

 so as, on the whole, to diminish during the whole time of 

 their passage across the disc ; thirdly, their behaviour will 

 be such that they reach a maximum at the centre ; and, 

 lastly, they will be found to increase in size during their 

 whole passage across the disc. These various types of 

 behaviour appear to us always to follow one another in 

 the above order ; and in a paper printed for private circu- 

 lation in 1 866, we discussed the matter at considerable 

 length, after having carefully measured the area of each 

 of the groups observed by Carrington, in order to in- 

 crease the accuracy of our results. In this paper we ob- 

 tained nineteen or twenty months as the approximate 

 value of the period of recurrence of the same behaviour. 



2. A recurrence of this kind is rather a deduction from 

 obser\'ations more or less probable than an hypothesis ; 

 nevertheless, it appeared to us to connect itself at once 

 with an hypothesis regarding sun-spot activity. "The 

 average size of a spot" (we remarked) "would appear to 

 attain its maximum on that side of the sun which is turned 

 away from Venus, and to have its minimum in the neigh- 

 bourhood of this planet." In venturing a remark of this 

 nature, we were aware it might be said, " How can a com- 

 paratively small body like one of the planets so far away 

 from the sun cause such enormous disturbances on the 

 sun's surface as we know sun-spots to be?" It ought, 

 however, we think, to be borne in mind that in sun-spots 

 we have, as a inaticr of fact, a set of phenomena 

 curiously restricted to certain solar latitudes, within which, 

 however, they vary according to some complicated peri- 

 odical law, and presenting also periodical variations in 

 their frequency of a strangely complicated nature. Now 

 these phenomena must either be caused by something 

 within the sun's surface, or by something without it. 

 But if we cannot easily imagine bodies so distant as 

 the planets to produce such large effects, we have equal 

 difficulty in imagining anything beneath the sun's sur- 

 face that could give rise to phenomena of such a com- 

 plicated periodicity. Nevertheless, as we have remarked, 

 sun-spots do exist, and obey complicated laws, whether 

 they be caused by something within or something 

 without the sun. Under these circumstances, it does 

 not appear to us unphilosophical to see whether as a 

 matter of fact the behaviour of sun-spots has any re- 

 ference to planetary positions. There likewise appears 

 to be this advantage in establishing a connection of 

 any kind between the behaviour of sun-spots and the 

 positions of some one prominent planet, that we at once 

 expect a similar result in the case of another planet of 

 nearly equal prominence, and are thus led to use our 

 idea as a working hypothesis. 



3. We have now a larger number of observations at our 

 disposal than we had in 1S66. We had then only the 

 groups observed by Carrington, the positions and areas of 

 all of which we had accurately measured. We have now 

 in addition five years of the Kew observations, for each 

 group of which the positions and areas have been recorded 



* By VlTarren De La Rue, U.C.L., F.R.S., Balfour Stewart, LL.D., 

 F.R.S., and Benjamin Loewy, F.R.A.S. Read before the Royal Society, 

 March 14, 1872. 



t Sefl Nature, vol. v., p. 192. 



by us in our previous communications to this society. 

 We have thus altogether observations extending from the 

 beginning of 1854 to the end of i860, forming the series 

 of Carrington ; and observations extending from the 

 beginning of 1866, forming the Kew series, as far as this 

 is yet reduced. We have, in fact, altogether a nearly 

 continuous series, beginning a year or two before one 

 minimum, and extending to the next, and thus em- 

 bracing rather more than a whole period. 



We propose in the following pages to discuss the be- 

 haviour with regard to size of the various groups of these 

 two series, as each group passes from left to right across 

 the sun's visible disc. Unfortunately for this purpose, a 

 large number of groups has to be rejected ; for, on ac- 

 count of bad weather, we have frequent blank days, 

 during which the sun cannot be seen, and on this 

 account we cannot tell with sufficient accuracy the 

 behaviour of many groups as they pass across the 

 disc. In our catalogue of sun-spot behaviour, we have 

 only retained those groups for which, making the times 

 abscissae, and the areas ordinates, we had sufficiently 

 frequent observations toenableustoconstruct a reasonably 

 accurate curve exhibiting the area of the group for each 

 point of its passage across the disc. From these curves 

 a table was then formed denoting the probable area of 

 each non-rejected group at the following heliographic 

 longitudes (that of the visible centre of the disc being 

 reckoned as zero) : — 



-63°-49°- 35'- 2i"-7°+7' + 2i°+35° + 49° + 63°; 

 in fact, giving the area of the group for the ten central 

 days of its progress, and rejecting those observations that 

 were too near the sun's border on either side, on account 

 of the uncertainty of measurement of such observations. 

 We have succeeded in tabulating in this manner 421 

 groups of Carrington's series, and 373 groups of the Kew 

 series up to the end of 1866, in all 794 groups. In this 

 catalogue the area is that of the whole spot, including 

 umbra and penumbra ; and in measuring these areas a 

 correction for foreshortening has always been made, as 

 described in a paper which we presented to this society, 

 and which constitutes the first series of our researches. 

 These areas are expressed in millionths of the sun's visible 

 hemisphere. 



4. When we began this present investigation into the 

 behaviour of spots, we soon found reason to conclude 

 that in the case of sun-spots the usual formula for fore- 

 shortening is not strictly correct. Perhaps if a sun-spot 

 were strictly a surface-phenomenon, the usual formula 

 might be correct, though even that is doubtful ; for the 

 earth as a planet may not impossibly affect the behaviour 

 of all spots as they cross the disc, so as to render the 

 formula somewhat inexact. However this may be, a spot 

 is probably always surrounded more or less by faculous 

 matter, forming in many cases a sort of cylindrical wall 

 round the spot. Now the effect of such a wall would be 

 to allow the whole spot to be seen when at or near the 

 centre of the disc, but to hide part of the spot as it 

 approached the border on either side. A spot thus 

 affected would therefore appear to be more diminished 

 by foreshortening than the usual formula would indicate ; 

 and we should therefore expect, if this were the case, that, 

 on the whole, after making the usual allowance for fore- 

 shortening, spots would nevertheless be found deficient in 

 area near the borders as compared with their area at the 

 centre of the disc. As a matter of fact we have some- 

 thing of this kind, as will be seen from the following 

 table, in which we have used the whole body of 

 spots forming the catalogue to which we have made 

 allusion. 



In this table the first column denotes the heliocentric 

 longitude from the centre of the disc reckoned as zero ; 

 the second denotes the united areas at the various longi- 

 tudes of all those groups from both series, the behaviour 



