ON MAPPING THE SURFACE OF THE MOON. 305 



The Table Land — Terra Photographica. — A few months ago Warren De 

 la Kue, Esq., presented to the Committee an enlarged photograph of the 

 moon (in four compartments), taken a few days after the first quarter, and 

 directed the attention of Dr. Lee and myself to an elevated tract surrounded 

 by the craters Cysatus, Curtius, Zach*, DeLuc, Maginus, and Clavius, which 

 he had discovered during the progress of his photographic experiments . . . 

 It is proposed, in accordance with Mr. De la Rue's suggestion, to designate 

 this Table Land ' Terra Photographica' (De la Rue). I have carefully com- 

 pared the region with Lohrmanu's and Beer and Madler's maps, and find a 

 portion of it very imperfectly represented by Beer and Miidler, with some of 

 the coordinates apparently misplaced. I have also obtained two series of 

 observations of its physical character. Under ordinary states of the atmo- 

 sphere, a few craters appear on its surface ; but on August 1, 1865, at Hart- 

 well, the surface of the plain was seen to be pitted with numerous small 

 craters ; and at a still later period I found that it consisted of a depressed 

 basin, with highly elevated rugged land abutting on Clavius, and an interest- 

 ing mountain-chain. Beer and Madler describe another mountain-chain, 

 which I have not seen, nor does it appear prominently on the photograph. 



The great Rill of Ariadceus. — -The Rev. T. W. Webb refers in his work on 

 Celestial Objects to a minute prolongation of this rill through the portion of 

 the Mare Tranquillitatis near Sabine and Ritter, detected by Gruithuisen 

 and Kunowsky ; a small portion only was seen by Beer and Madler. I 

 succeeded in observing it on October 20, 1864, and have seen it since. It 

 has also been seen on two or three occasions since October 1864 by Mr. 

 Freeman at Mentone, in the Alpes Maritimes. Mr. Freeman has traced it to 

 the group Ritter, Sabine, &c. 



Among the observations of an instrumental character, I may mention some 

 " that are differential of the second order, and which have reference to five 

 groups now under systematic observation, both physically and instrumentally, 

 as the state of illumination and other circumstances may allow, viz., the 

 Picard group, the Posidonius group, the Dionysius group, the Gassendi group, 

 and the Triesnecker group, each crater (or in some cases mountains within 

 them) forming a central point ; the angles and distances of each measured 

 object being referred to a standard line, the coordinates of the two extremi- 

 ties having been determined by Beer and Madler. 



APPENDIX. 



Translation of a Letter from the Director of the Observatory at Athens, 

 J. E. Julius Schmidt, to Mr. W. R. Birt, London. 



Your favour of the 23rd of September with which you honoured me is in 

 my possession, t beg leave to answer your inquiry at once and in general, 

 partly to show a mark of my sympathy for the interest you take concerning 

 the topography of the moon, partly to make known to you and others the 

 limits within which I have been occupied on the same object during a quarter 

 of a century. 



Passing over in silence my observations before the year 1842, I allude 

 only to those which afford a topographical interest, and that are at present of 

 some utility to me. Since June 1842 I could employ more powerful tele- 

 scopes, and the sketches of detailed landscapes of the moon were obtained 



* A crater between Zach and De Luc, perfectly visible in the photograph, but not on 

 Beer and Madler's map, forms part of the boundary of the "Terra Photographica." It 

 is proposed to name it " Pollock, C.B." 



1865. Y 



