2 MEMORIE DELLA SOCIETÀ' 



will probably remain uunsual. The annexed steel engraving (tab. L1X) from studies 

 iliade at the Allegheny Observatory, chiefly with the full aperture (13 English inches) 

 of its Equatorial has been prepared by the Kind furtherance of Prof. George F. Barker 

 of Philadelphia its execntion being seenred at the hands of au engraver who has 

 done bis work with peculiar fidelity and skill. I trust it will be accepted as a 

 means of putting the reader in a certaiu seDse in the place of the observer, and 

 enabling bim to himself directly compare theory with the faets of observation. 

 It is from sketches taken on the 23 ld 24 th and 25 1 " of december 1873 of the 

 eastern extremity of the great spot then nearing the centre of the San, and about 

 12° south of the solar eqnator. It is called a « typical spot » because (siuce the 

 details could not be compieteci at a sitting), it is Iess an accurate outline ofwhat 

 could be seen at any one moment, than an assemblage of the different types it 

 presented, in their proper connection. The whole then is taken from observation, 

 but while the details of the photosphere etc. have been supplemented from other 

 studies , everything in the raain body of the spot is the most literal trauscript 

 I could make of specific penumbral and umbral forms. 



The Sun had been hidden bere for some days before the 23 rd of december, when 

 the sky cleared disclosing a spot of more than usuai size. Although a daily re- 

 cord of the solar surface is maintained at the Allegheny Observatory the weather 

 for some weeks before had iuterrupted it so capriciously that I am unable to say 

 with certainty what the age of the spot was when it suddeuly presented itself. 

 Unquestionably however it had at this time already passed through the initiatory 

 stage of its formation, and had entered on that in which the forms seen earlier, 

 have commenced to become segraented or distorted, while stili retainiug character- 

 istics which show the type from which they have sprnug. (A). 



Attention was first directed to that dark interior , in which Dawes discerned 

 stili darker shades, he called nuclei; as the nnusual size of the spot and the ir- 

 regularity of shade in the umbra, seemed to favor their iuvestigation. Aided by 

 special optical devices, there became visible to dose attention, forms which ap- 

 peared to be affiliated to the better known oucs of the peuumbra , which Avere 

 studied also, and a description of a part of whose characteristics, interesting pe- 

 rhaps in their hearing npon solar theories follows. (B). 



It was observed. 



1. That the now well known « filameuts » of the penumbra, and those (stili to 

 be described) of the umbra, were ali disposed in curves. These curves might be 

 described in general as portious of rude spirals, since while there was such ava- 

 riety as to make classification difficult, this spirai type was as a whole, beyond 

 any question the dominant one. This and the characteristic forms of the outer pe- 



