﻿184 Mr. W. R. Birt on some recent Investigations 



accumulated I found myself in a position to attempt a discussion 

 of them, and adopted the following method :— A ledger contain- 

 ing as many columns as spots having been prepared, each obser- 

 vation of each spot was posted to its appropriate column, and a 

 series of numbers obtained which represented the " visibility f 

 of each : for example, the spot No. 1, nearly centrally situated, 

 was seen more frequently than any of the others ; the number of 

 observations, therefore, would be a measure of its comparative 

 high visibility. Taking this spot as the standard, the ratios be- 

 tween the observations recorded of it and those of the other spots 

 respectively would be the degrees of visibility appertainingto each. 

 The following Table contains the degrees of visibility of each spot 

 as determined from the observations of twenty luuations :— 



No. 



Obs. 



Vis. 



No. 



Obs. 



Vis. 



No. 



Obs. 



Vis. 



0. 



10 



•044 



13. 



34 



•148 



25. 



33 



•144 



1. 



229 



1-000 



14. 



99 



•432 



26. 



1 



004 



2. 



9 



•039 



15. 



4 



•017 



27. 



2 



•009 



3. 



207 



•904 



16. 



67 



•293 



28. 



1 



•004 



4. 



204 



•891 



17. 



192 



•838 



29. 



8 



'035 



5. 



121 



•528 



18. 



19 



•083 



30. 



38 



•166 



6. 



49 



•214 



19. 



31 



•135 



31. 



6 



026 



7. 



24 



•105 



20. 



9 



•039 



32. 



16 



•070 



8. 



3 



•013 



21. 



5 



•022 



33. 



3 



•013 



9. 



50 



•218 



22. 



39 



•170 



34. 



5 



•022 



10. 



13 



•057 



23. 



11 



•048 



35. 



1 



004 



11. 



33 



•144 



24. 



11 



•048 



36. 



1 



004 



12. 



6 



•026 















The agencies known to affect the appearances of lunar objects 

 are illumination, reflection, and libration, which are more or less 

 regular in their operation ; consequently the variations in visi- 

 bility dependent upon them should also be regular, and exhibit 

 phenomena of periodicity. Another and much more powerful 

 agency in affecting the visibility of such objects as we are now 

 dealing with consists in the variable conditions of our own atmo- 

 sphere, which are far from being regular in their occurrence. 

 Before we can arrive at a sound conclusion on the phenomena 

 presented by the spots, it is necessary to eliminate the effects of 

 these agencies ; and no better mode presents itself than that of 

 obtaining a long series of observations, during the period of 

 which opposite effects are likely to be compensated and differ- 

 ences arising from instruments, observers, &c. neutralized. 



In order to ascertain, if possible, the agencies affecting the 

 spots, the degrees of visibility have been computed for every pair 

 of lunations from April 1869 to November 1870 inclusive, and 

 the results projected in curves. Upon the supposition of no 

 other agencies than those above named affecting the spots, we 



