OBSERVATIONS OF LUNAR OBJECTS. 245 



Report of the Committee for discussing Observations of Lunar Objects 

 suspected of Change. The Committee consists of the Rev. T. W. 

 Webb, the Rev. Robert Hakley, F.R.S., and Edward Crossley, 

 Secretary. 



•The Committee have pleasure in presenting their Second Eeport on the above 

 subject. It will be remembered that the Report of last year was confined 

 principally to the discussion of the possible variations of visibility of the 

 numerous spots and craterlets upon the floor of Plato under the same condi- 

 tions of illumination. That now presented is directed chiefly to the discus- 

 sion of the various streaks aud bright patches which interlace the spots and 

 craterlets. 



One interesting and important change ha^ been fairly shown — the floor of 

 Plato becomes darker with the increase of the sun's altitude. Mr. Birt has 

 suggested an explanation of this phenomenon. Whatever be the true cause 

 of this change, it is very dijBficult to account for it by the ordinary laws of 

 reflection. ^Yhen we consider the varying aspect of the streaks at the same 

 time of the luni-solar day, we cannot but think that, with careftil observations 

 made with powerful instruments, such as the Newall Refractor and many 

 others, we may be able to confirm or otherwise a physical explanation of 

 these curious changes involving the existence of certain gases aud vapours 

 upon the surface of the moon. 



The Committee can only look upon the study of Lunar Physics as in its 

 infancy, and they trust that in future years the Association will not overlook 

 this important branch of astronomical inquiry. 



Eeport on the Discussion of Observations of Streaks on the Surface of the 

 Lunar Crater Plato. By "W. R. Birt. 



In completing the task assigned to me of discussing the observations of the 

 streaks on the floor of Plato, I have been desirous of including every, eVen 

 the most minute, circumstance bearing on the exhibition of phenomena that 

 may possibly illustrate the condition of a small portion of the moon's surface 

 at the epoch 1869 April to 1871 April. Drawing my conclusions from the 

 experience of twelve years, I feel that I may confidently say it will be some 

 years before another series of observations of a particular region will be 

 undertaken with the view of so closely examining the spots and streaks cha- 

 racterizing it, unless a staff of eflicieut observers be organized with the pro- 

 vision of a fund sufficiently ample to defray all the necessary expenses. The 

 work is a difficult one. The staff should consist of not less than six devoted 

 observers, who would, independently and most probably, as in the present 

 case, work with instruments of varying aperture and carefully record all 

 their observations. The principal qualification is a keen eye for the apprecia- 

 tion of delicate variations of tint, and the detection of minute spots of light 

 with a readiness of referring them by estimation and ahgnment to the respec- 

 tive localities of the region on which they are seen. The observations should 

 not be allowed to accumulate, but should be forwarded at once to an experi- 

 enced selenographer charged with the work of arranging and discussing them. 

 Taking into consideration the results of the discussion of the present and 

 previous years embodied in the two Reports, it appears that in order to con- 

 firm these results, and to open up new investigations in other regions of the 

 moon's surface, the requisite time cannot well be fixed at less than three 

 years — five would most probably afford the best results. 



