ox MAPPING THE SURFACE OF THE MOON. 11 



especially of the highest part, a little south of Linne. I E^ ^ [y Posidoiiius], 

 when near the morning or evening tenuinator, shows itself as a distinct 

 mountain peak of 150 toises, or 959-2 English feet in height. It is only 

 when the sun attains a considerable altitude on y that it presents the same 

 appearance as Linne, viz. that of a white diffused cloudy patch. So far as I 

 am aware, it is only recently that this similarity of appearance between these 

 objects has been observed. Although many monntains and craters lose their 

 distinguishing featnres, and appear as round white spots when the sun is at 

 a great altitude above their respective horizons, there are numerous craters 

 that present the characteristic appearance of having a darJc interior, sur- 

 rounded by a hriglit ring under the more direct rays of the sun, when most 

 mountains are seen as bright spots. 



Connected with the similarity of appearance under high illumination is 

 another interesting feature characterizing Linne and y Posidonius [I E* '^], 

 ■\"iz. the existence during the period of observation, of crater-openings on 

 both. Of that on Linne I E')' ^ we have numerous records. That on y [I E^ ^] 

 is certainly smaller than I E^ ", and has been seen only on five occasions. 

 It was discovered 1867, Januaiy 14, by Mr. Knott, with his T^-inch 0. G. 

 by Alvan Clark. His own words wiU best describe the nature of the disco- 

 very. "Writing under date of March 3, 1867, he says, " While observing 

 Linne' on the 14th of Januaiy, at about lO** 30" G. M. T., I had myself a 

 strong impression of a darl: sj^oi, as described by Schmidt, but definition was 

 so poor, and I saw, or fancied I saw, traces of a simiiar appearance on Posi- 

 donius y, that I regarded it as an illusion, and made no note of it at the 

 time. I could not, however, free my mind from the idea that there mir/lit be 

 something in it, and accordingly, two days afterwards, I added the following 

 note, which I transcribe verhatim : — 



" I had a verg strong impression, with various eyepieces, of a small cen- 

 tral dark spot on the diffused patch covering (?) Linne, so strong that I 

 inclined to regard it as having a real existence ; as, however, I saw a similar 

 appearance, though not nearly so strongly marked, on y [Posidonius], I can 

 onlj" regard it as a curious optical illusion." — Xote added Januaiy 16, 1867. 



Tliis dark spot on y Posidonius was next seen by Mr. Buckingham on the 

 11th of April, 1867. His observation is thus recorded :— 



" 1867, April 11, 6"^ to 10'> 59™. Air very steady, but slightly hazy, and 

 found y Posidonius a fine crater, 0"-5, seen well with 360 and higher, 

 clearly with 250, but could not with 120." 



1867, May 11. Herr ScinriDT recorded as follows : — " I also see a deli- 

 cate black point in y Posidonius ". 



1867, Oct. 16. Mr. BucKiNGnAM saw and described it as very black. 



1867, Dec. 4, 7"^ 30™, G. M. T. Mr. Knott records that it was well seen*. 



The increase of the brightness of Linne as the sun attains a greater alti- 

 tude above its horizon, especially as y Posidonius does not exhibit it in so 

 marked a degree, may bear a passing remark without at all hazarding an 



* About the middle of January 1866, Mr. Leigh, then of Birkenhead, novr of War- 

 rington, detected a curious group of three small craters and three smaU moimtains north 

 of Aristoteles, which is figured by Webb in the ' Intellectual Observer,' Ko. 60. vol. x. 

 Jan. 1867, p. 441. April 11, 1867, Webb detected on the western of the three mountains 

 a shallow pit (I I' ") (see ' Intellectual Observer,' l^o. 64. vol. si. May 1867, p. 282). Webb 

 h.id pri->viniisly (1806, April 21) detected a cavity or pit (T I**) on tlie western mountain 

 of B. & M.'s r nortli of Ari.stoteles, which was extremely plain 0]i April 11, 1867. In 

 connexion with the phenomena presented by Linne, the value of observations of these and 

 similar object.'^ is obvious. 



