500 REPORT — 1873. 



Again, in connexion with these solar researches, it is of importance to 

 know both the lieating and actinic effects of our luminary, and how these 

 vary, not only from hour to hour, but from day to day and from year to year. 



No instrument has, however, yet been devised by which the heating-effect 

 can be conveniently registered. On the other hand, Dr. Roscoe has perfected 

 his method of observing the actinic effect so as to make it automatic ; and 

 thus a series of hourly observations of this element of the sun's activity can 

 be very easily obtained. This ought to be done at every station where the 

 surface of the sun is mapped ; and we understand that this plan of Dr. Roscoe's 

 is about to be adopted in all Russian observatories. It would thus appear 

 that we are now in a position to define with precision what ought to be done 

 at a sun-station ; and, as long as the sun-establishment at Kew lasts, ob- 

 servers may there receive instruction in solar photography through the 

 courtesy of Mr. De La Rue. 



They may also receive instruction in the art of measuring the areas and 

 position of sun-spots through the same source ; and, finally. Dr. Roscoe will 

 be glad to give the necessary instruction in actinic observations. 



It is hardly necessary to remark that the stations should be so selected as, 

 taken together, to be independent of weather, and to be capable of giving at 

 least one picture of the sun's disk every day without the chance of inter- 

 ruption. We know enough of the climate of various places to bring about 

 this result ; and in our dependencies, if not in Great Britain, we have a suffi- 

 cient area from which to choose our stations. 



The influence of weather in causing blank days is particularly detrimental 

 in solar research. In the observations lately reduced by Messrs. De La Rue, 

 Stewart, and Loewy, it has been found that a good record of the behaviour of 

 sun-spots, with regard to increase and diminution, as they pass across the 

 disk, is of great value ; but that, owing to blank days, this record can only be 

 obtained for half the whole number of spots observed, and even for this half 

 in a more or less imperfect manner. And it is of so much greater importance 

 to select the stations so as to obtain a conluiuous record, inasmuch as such 

 observations are not like experiments which may be multiplied ad libitum ; 

 for here we are furnished in a year with a record of a certaiu number of sun- 

 spots and no more ; and it remains with us to make the best possible use of 

 the limited information which nature gives us. 



In fine it is believed that a daily record of the sun's surface, accompanied 

 by a record of his actinic power, is, in the present state of science, of the 

 greatest possible importance. 



In the preceding remarks no allusion has been made to the establishment 

 of regular spectroscopic observations of the sun's disk — not because it is con- 

 sidered unimportant, but because it forms a separate branch of inquiry, which 

 will be best reported upon by Messrs. Janssen and Lockyer, and by Dr. Hug- 

 gins, gentlemen who have especially devoted themselves to this subject. 



Your Committee have received the following communication on the im- 

 portance of the establishment of regular Spectroscopic Observations of the 

 Sun's Disk from Mr. Lockyer. 



Mr. Lockyer^s Statement. 



The following are some among the secular inquiries which in my opinion 

 ought to be undertaken at once on a perfectly definite basis and" with un- 

 sweiving regularity. Of course I have not named all the secular inquiries, 

 nor have I alluded to any of the special ones which are suggested almost 



