57o 



NA TURE 



[April 16, 1903 



1 «o- 



The result of the discussion showed that the curve re- 

 presenting the variation of percentage frequency of the 

 prominences for the whole limb of the sun indicated that 

 in addition to the main epochs of maxima and minima 

 coinciding in time generally with those of the maxima and 

 minima of the total spotted area, there were also prominent 

 subsidiary maxima and minima. 



Further, dividing the sun's limb into zones of 20 in 

 width from the equator, with a pole zone of io°, and dis- 

 cussing each zone separately, the variation of the promin- 

 ence percentage frequency about the equator was found to 

 be very different from that in the higher latitudes, the 

 former changing with the spots, and the latter exhibiting 

 sudden outbursts just previous to the epochs of sunspot 

 maxima, followed and preceded by comparatively long 

 intervals of quietude. 



In the present investigation the prominence observations 

 have been discussed from a different point of view, in i rder 

 to trace out, if possible, the heliographic latitudes of the 

 chief centres of action of prominence disturb. im e, or in 

 other words, to indicate the regions on the solar disc where 

 prominences were most prevalent in each year, and see if 

 those regions varied their positions in relation to the sun's 

 equator. 



In this way it could be determined whether such move- 

 ments, if any, are subject to some periodic law, in which 

 case it would be possible to increase our knowledge of the 

 circulation of the solar atmosphere in regions outside those 

 in which sunspots alone have, up to the present, been 

 employed. 



It has long been known that the centres of action of sun- 

 spot disturbances, as shown by Carrington, Spoerer and 

 others, are restricted to particular- 

 regions on the solar surface, all of *,_ 

 which are included in the two large 

 zones from +5° to + 35° heliographic 

 latitude. Further, from year to year, 

 the regions of greatest activity undergo 

 changes of position which are periodic. 

 Thus at sunspot maximum there is only 

 one zone in each hemisphere in which 

 spots are situated, the centre of this be- 

 ing about 18 N. and S., while at mini- 

 mum there are two zones existing 

 simultaneously in each hemisphere ; the 

 older cycle dying out in the zone the 

 centre of which was situated in low 

 latitudes, and the new one commencing 

 in high latitudes, its centre being about latitude +30 t<> 



±35°- 



It may be here remarked that the above results are not 

 strictly, but only generally, true, because the observations 

 of each solar hemisphere have not been treated sufhcientlv 

 in detail. If this be done by examining the behaviour of 

 the frequency or areas of spots in, say, zones of 5° in width, 

 then it will be found that sometimes there are actually 

 three centres of spot activity. The reduction of sunspots 

 in this manner for the whole period, since accurate measure- 

 ments have been made, is not yet complete, but it is as 

 well to draw the reader's attention to these facts. 



Fortunately, the investigator has at his disposal two 

 splendid series of observations of prominences made in- 

 dependently of each other, so that he is able to check the 

 variations indicated in one series by seeing if they are ex- 

 hibited in the other. 



The observations thus discussed were made by Tacchini 

 at Rome from 1872-1900, and by Rirco and Mascari at 

 Catania from 1881 to 1901. Both sets of observations are 

 handled in exactly the same way, and it will be seen later 

 that the changes indicated in each are practically identical. 

 It is due to the kindness of Prof. Ricco, who forwarded some 

 unpublished data concerning his prominence observations 

 and deductions, that the curves are complete up to the end 

 of the year 1901. 



The method of reduction adopted was to determine for 

 each year the percentage frequency of prominence activity 

 for every 10 degrees of solar latitude north and south. A 

 series of curves was next drawn, one for each year, the 

 absciss.-e representing the latitudes of prominences north 

 and south, and the ordinates their percentage frequency. 



NO. 1746, VOL. 67] 



It was then found that the centres of prominence activity. 

 or in other words, the maxima of the curves were sometimes 

 single, sometimes double, and in one or two cases even triple 

 in each hemisphere. This suggested that just as some- 

 times there are two zones of spots existing at one time, 

 so there might be one, two, or occasionally three zones of 

 prominences in existence in each hemisphere simultaneously. 



Further, a close examination of the whole set of curves 

 with reference to these points of maxima made it possible 

 not only to study the changes of latitude of these points 

 from year to year, and their positions when commencing 

 to develop or about to disappear, but the intensity of these 

 centres in relation to each other. 



The accompanying illustration (Fig. 1) shows the curves 

 drawn for the years 1879, 1880, and 1881, from the observ- 

 ations of Tacchini, and serves as examples of the curves 

 that have been discussed ; they exhibit the change from a 

 single to a double centre of activity in each hemisphere. 



'1 hus, in 1879, there was a prominence maximum in each 

 hemisphere at latitudes ±50°. In the next year (1880), 

 both these maxima had retreated further away from the 

 equator, namely to latitudes ±6o°, while another centre of 

 disturbance began to make itself apparent at latitudes +30°. 

 In the year 1881, both centres in each hemisphere were 

 strongly marked and became of about the same intensity, 

 their mean latitudes in each hemisphere being about ±30° 

 and + 60°. These curves thus indicate that during these 

 three years, the direction of motion of these centres of 

 activity tends polewards or away from the equator. 



By examining both series of observations made by 

 Tacchini and Ricco and Mascari, and analysing the posi- 

 tions of the principal and subsidiary maxima for the whole 



tai3 



200 300 400 6O0 



taao 



100 200 300 400 600 



Ififll 



100 200 300 -K10 500 (IK 



Fig. 1. — Curves illu' 

 of heliocentrii lati 



.trating the percentage frequency of solar prominences for each 10 degrees 

 tude tor I he years 1S79, 1880 and i88r : aller the observations of Tacchini. 



period covered by the observation--, the results illustrated 

 graphically in Fig. 2 were obtained. 



In these figures the facts are brought together for each 

 hemisphere separately. The medials of the lines (curves 

 A and B) show the heliographic latitudes of the centres of 

 prominence action ; the thickness of these lines represents 

 the relative percentage frequency of prominence action. 



For the sake of comparison, three other curves for each 

 hemisphere are shown. The first curves (C) show the mean 

 heliographic latitude of spotted area for each hemisphere; 

 these curves, as previously pointed out, are only generally 

 accurate. The next curves (D) illustrate the variations of 

 the percentage frequency of prominence action for each 

 hemisphere taken as a whole, and are similar to those given 

 previously. 



The last curves (E) show the variation of the mean daily 

 area of sunspots from year to year, also for each hemi- 

 sphere. 



With this diagram the reader will be able at once to 

 compare the variations of the changes of latitude of 

 prominences as determined from the Roman and Sicilian 

 observations. He will also be able at a glance to correlate 

 these variations with those exhibited in the other curves 

 added for comparison. It will therefore suffice if a sum- 

 mary of the conclusions drawn be given. 



(1) The centres of action of prominence activity undergo 

 an apparently regular variation. 



(2) The direction of motion of these centres is from low 

 to high latitudes, the reverse of that of spots, which travel 

 from high to low latitudes. 



This is seen directly from the curves, the prominences 

 beginning in about latitude +20 , and moving away from 



