KNOWLEDGE & SCIENTIFIC NEWS. 



[Jan., 1905. 



to a comparatively narrow zone near each side of the 

 solar equator, while there seems no such restriction to the 

 former. Again, if the relation between the areas of spots 

 and prominences be considered, those of the former are 

 practically insignificant. A study of prominences is 

 therefore of the highest importance when tiie activity of 

 the solar atmosphere is in question, but, unfortunately, 

 records of these only commence in the year 1870. Unlike 

 sf)ots, which, as previously pointed out, are the result of 

 the descent of comparatively cool matter from the upper 

 regions of the solar atmosphere, prominences consist of 

 ascending currents of highly-heated matter from the lower 



a»40-135 I8«J-S40 



Fig. 5. — If the areas of all the sunspots which appear on the Sun's disc 

 minimum to the next be added together, then the abDvc squares 

 relative change of spotted area for each of the periods from the year 



to the higher layers; indeed, tliey are the precursors of 

 spots and are thus more direct indications of solar disturb- | 

 ances. That they are very important factors in solar 1 

 " weather " can be gathered from their enormous magni- 

 tudes.someof them being 100,000 miles or more in height 

 and correspondingly broad. 



Prominences, like spots, ha\e periods of maximum 

 and; minimum frequency. As a rule, when there are 

 few , spots there are few prominences, and when the 

 spotted area is large so is tliat of the prominences. 

 '1 here is thus a very close connection between these two 



Ht- <>■- A picture of the Sun taken in light of one colour, showlni: 

 that there are other areas on the solar disc which are more 

 eitentlve than those of spots. The former appear bright while 

 the latter are dark. 



phenomena ; but it must be stated that this connection 

 only holds good when the prominences situated nearest 

 the equatorial regions of the sun are alone taken into 

 consideration. It is due, however, to the fact that pro- 

 minences are at times very numerous near the solar 

 poles that the curve representing the mean variation of 

 their frequency from year to year does not rise or fall 

 gradually throughout a cycle, but is of a wavy nature, as 

 can be seen by a glance at the curve shown in a subsequent 

 figure (fig. 7). It IS this peculiarity that makes the promi- 

 nence curve so important, for these "humps" on the main 



curve represent solar changes of activity that are scarcely 

 traceable on the spot curve. There seems reason to 

 believe, therefore, that the observations of prominences 

 are capable of giving us far more information regarding 

 the circulation and activity of the solar atmosphere than 

 those of spots. 



We thus see then that the study of spots and promi- 

 nences has made us acquainted with three different 

 periods of solar changes. Thus we have a short period 

 of a litlle less than four years, another cycle covering in 

 the mean a little more than eleven years, while a third 

 variation occupies about thirty-five years. 



Having thus briefly summarised the chief 

 facts concerning the various changes of solar 

 activity, attention will now be paid to the 

 records of meteorological phenomena to see if 

 any trace can be found corresponding to these 

 solar variations. The question now arises as 

 '~ "" ■ to which meteorological element should be 



chosen to commence operations with. For 

 show the several reasons, which need not be mentioned 

 '834. here,barometricobser\ationshavebeen selected, 



for they supply us with an excellent means 

 of detecting variations of pressure which are direct indica- 

 tors of air movements towards or away from the earth's 

 surface. Greater solar radiation means greater heating 

 power, and therefore stronger ascending currents away 

 from the earth in some parts of the world, and conse- 

 quently greater descending currents in other parts ; thus 

 we should expect to find lower and higher pressures 

 simultaneously in different regions of the earth's surface. 

 A decided great advantage in employing barometric 

 records is that the variations of this element from year to 

 year are very similar over large areas, and do not change 

 according to local conditions as is the case with rainfall. 

 Thus, to take the case of the British Isles, for example, 

 the pressure variation of, say, Oxford is quite sufficitnt 

 to illustrate the variation o\er the whole of the British 

 Isles, as if we employed the records of N'alencia, .Aber- 

 deen, Greenwich, or Edinburgh, which are all quite 

 similar. Rainfall is the effect and not the cause of baro- 

 metric pressure variations, and we in these islands are 

 quite familiar with this fact. A fall in the barometer 

 with us generally means rain, and a rise probably dry 

 weather. Rainfall then being an after-effect of pressure, 

 any variation of the latter should have a very close 

 connection with the former. 



{lo be continued.) 



Messrs- Adolph's Selenivim Cells. 



Wc have received from Messrs. Adolph, of Farringdon 

 Koad, a catalogue of their selenium ceils; and selenium 

 cell apparatus. The great interest of selenium to the cleclri- 

 cian lies, as everybody knows, in the alteration which becomes 

 apparent in the electro-conductivity of this element as the 

 light thrown on it varies. The relation between the change of 

 electric resistance and the amount of illumination has been 

 expressed mathematically ; and if selenium could always be 

 depended on to behave with perfect regularity, some of the 

 practical uses to which it could be put might effect astound- 

 ing revolutions in light telephony. Hcrr Kuhmer, in Herlin, 

 has endeavoured to transmit sounds along beams of light by 

 the employment of selenium cells, and within ccrf.iin limits 

 the experiments have been successful. What is wanted most, 

 however, with regard to selenium, isncw and continued experi- 

 ment ; and the opportunity which Messrs. Adolph aflord of 

 bringing within the reach of laboratory students selenium 

 cells of all kinds, as well as apparatus for testing its properties 

 in light telephony, and its applications to other branches of 

 research, is one that wc warmly welcome. 



