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NATURE 



\Mar. 28, 1872 



of which we have been able to obtain with accuracy ; 

 while the third column exhibits the residual factor for 

 foreshortening, which will bring the areas of the second 

 column into equality with each other. 



Table I. 



5. From the above table it appears that the average 

 behaviour of spots, as far as can be judged from the infor- 

 mation at present attainable, is not quite symmetrical as 

 regards the centre of the disc. Without attempting at 

 present to enter into an explanation of this remarkable 

 phenomenon, we may point to it as a confirmation of our 

 view previously stated, that most spots are accompanied 

 by a wall-shaped surrounding of facula. Observations 

 show that on the whole the life-history of the facula begins 

 and ends earlier than that of the spot which it surrounds, 

 and that throughout a gradual subsidence of this elevated 

 mural appendage seems to be taking place. But such a 

 diminution of the wall discloses more of the spot itself, 

 and hence the spot-areas, measured on the eastern half 

 of the hemisphere, might be expected, Cictcris paribus, to 

 be smaller than those observed in the western half, a fact 

 strikingly demonstrated by the above table. 



Our present object, however, is not to account for the 

 average behaviour of spots, but rather to investigate the 

 causes or concomitants of a departure from this average 

 behaviour. We have, therefore, in all cases made use of 

 the factors given in the above table as those which, 

 judging by the average behaviour, tend to equalise the 



areas that pass the various longitudes. We have called 

 this carth-corrcctioii, and have limited our discussion to 

 any well-marked behaviour that remains after the earth- 

 correction has been applied. 



Let us now divide the whole mass of observations into 

 four portions, depending upon the position of the planet 

 Venus with reference to the earth or point of view. First, let 

 us take each occasion on which the planet is in the same 

 heliographic longitude as the earth, that is to say, when 

 the earth and Venus are nearly in a line on the same side 

 of the sun. 



Let us use five months' observations for each such 

 occasion, extending equally on both sides of it ; thus, for 

 instance, if the planet Venus and the earth had the same 

 heliocentric longitude on September 30, 1855, we should 

 make use of sun-spots from the middle of July to the 

 middle of December of that year as likely to represent 

 any behaviour that might be due to this particular posi- 

 tion of Venus. Let us do the same for all similar occa- 

 sions, and finally add all the spots thus selected together. 

 We have thus obtained a mass of observations which may 

 be supposed to represent any behaviour due to this posi- 

 tion of the planet Venus with reference to the earth or 

 point of view. 



Secondly, let us now take each occasion on which 

 Venus is at the same longitude as the extreme right of 

 the visible disc, that is to say, 90° before the earth, and 

 do the same as we did in the previous instance, using five 

 months' observations for each occasion We shall thus, 

 as before, obtain a mass of observations which may be 

 supposed to represent the behaviour due to a position of 

 Venus go° before the earth. Thirdly, let us obtain in a 

 similar manner a mass of observations representing the 

 behaviour of sun-spots for a position of Venus 180^ before 

 the earth, Venus and the earth being now at exactly 

 opposite sides of the sun ; and fourthly, let us finally 

 obtain, in a similar manner, those observations represent- 

 ing the behaviour of sun-spots when Venus is 270° before 

 the earth, being now of the same heliocentric longitude as 

 the extreme left of the visible disc. 



These four series of five months each will in fact split 

 up the whole body of observations into four equal parts, 

 the synodical revolution of Venus being nearly twenty 

 months. The following table exhibits these series after 

 the earth-correction has been applied to each. It also 

 represents each series reduced so as to exhibit its charac- 

 teristic behaviour for an average size of spot = 1000. 



7. We may do the same for the planet Mercury as we 

 have done for Venus, that is to say, we may spht up the 

 whole body of observations into four parts, representing 

 the behaviour of sun-spots when Mercury is in the same 



four positions with respect to the earth as those which are 

 given for Venus in the above table. Only in this case we 

 must bear in mind that, owing to the eccentricity of 

 Mercury's orbit, this planet will sometimes take a longer, 



