Prof. Secchi on Solar Spots. 495 



ments of scientific instructiou. The subject, however, is so 

 remote from practical apphcatiou, that there is httle hope of any 

 impression being made in such quarters for many years. In the 

 mean time it seems the duty of those who have profited by it, 

 to do what lies in their power to proclaim its merits and acknow- 

 ledge the value of the idea first struck out by Mr. Herapath, and 

 perhaps saved from oblivion by th^ Philosophical Magazine of 

 that period. ' ' • <. 



Edinburgh, Nov. 6, 1858. * '-- ' - 



LVI. On Solar Spots, and the MethJbd. of determining their 

 Depth. By Father Angelo SECCuil'JP.r of essor of Astronomy/, 

 and Director of the Observatory of the floman College^. 



IT seems that the attention of astronomers and natural philo- 

 sophers has been now turned again in a special manner to 

 the study of the chief luminary of our system, — while some time 

 ago it appeared to be abandoned, as though they feared to be 

 overcome by the force of such a mass of light. 



Prom the imwearied attention of men of science much has 

 been already discovered in the sun, and there is a hope of our 

 yet knowing much more concerning it ; it is therefore with plea- 

 sure that we see this subject followed up with general ardour, 

 particularly after the result of the recent experiments on the 

 solar temperature. 



The most interesting question is that of the nature of the 

 solar spots, with respect to which, although a great number of 

 natural philosophers are agreed in admitting them to be gaps in 

 the solar atmosphere, yet from time to time the old hypotheses 

 respecting them are revived, which suppose them to be clouds, or 

 something else indefinable ; and it is for that reason that I deem 

 it to be fitting not to recede from what was established by some 

 of my recent observations, which tend to subvert for ever all 

 those ancient hypotheses that took their origin only from the 

 inferior quality of the instruments with which the observ-ations 

 were made, or from want of experience on the part of the ob- 

 servers themselves. 



The luminous coating of the sun, to which the name pho- 

 tosphere has been given, is without doubt in a fluid state, as is 

 shown by its prodigious instability ; and the spots are nothing 

 else than gaps in this coating, that allow of the sensibly dark 

 body of the sun being seen. In another memoir of mine con- 

 tained in the Acts of the Academia di Nuovi Licei, I have dwelt 

 strongly upon this point, the fundamental idea of which is not 



♦ From the Astronomische Nachrichten, No. 1148; coinmuiiiciited by 

 W. G. Lettsoin, Esq. 



