OBSERVATIONS OF LUNAR OBJECTS. 253 



completed, very strange in comparison with last night's sketch. Its form, 

 complicated as it was, was very carefully traced in the drawing, and repeat- 

 edly seen afterwards, but so delicate that it was impossible to see the whole 

 at once. It required to be traced out by minute attention." Compare Mr. 

 Pratt's drawings (figs. 5, 6, and 7) with Mr. Gledhill's of September 25 (fig. 

 9, jposi,. p. 263). 



In comparing Mr. Pratt's sketches of August 16 and 17, the transition in 

 the 25 hours is very remarkable ; indeed so much so as to indicate that some 

 extraordinary change had come over the floor in the interim : the disposition 

 of light and dark is almost entirely diff'erent, yet I think I can trace the 

 effect of " action " on the floor. On the 16th the lighter portion affected the 

 border, the darker portion being entirely separated from it. The northern 

 boundary of the dark portion took the form of the streak c and k, which was 

 plainly seen on the 17th, with a dark portion on its north. Had the streak 

 c and K a motion southward from the north border between the two observa- 

 tions? Again, on the 17th, e, the N.W. arm of the trident, was not sepa- 

 rated from c. It would appear that the markings, as seen on the 17th, were 

 evolved, from the peculiar and remarkable disposition of the light and dark 

 portions seen on the 16th. It will be seen further on that, as the observa- 

 tions proceeded, the light portion was not unfrequently noticed to be in con- 

 tact with the north border. 



1869, August 20. — Mr. Gledhill described the floor as darker than the 

 surface of any of the i¥arja=0-99 ; the sector faint and all spots faint. On 

 the same day Mr. Pratt thus graphically describes his seeings: — "The 

 shading on the floor of Plato is quite a study, and a perplexing one ; some- 

 times, when the air is disturbed, a light sector (S.E. part), like Mr. Birt's 

 key-plan, is alone visible. Again, in a few minutes two streaks from Anaxa- 

 goras would seem to cross the floor, as sketch May 22, 1869 ; then, again, 

 between the two, a third narrower streak appeared, similar to Webb's copy in 

 ' Celestial Objects ; ' and as definition improved, a light marking near the north 

 rim {k) was seen ; and again a change, and the appearance is decidedly the 

 same as on the 17th of August, a near approach to Mr. Knott's sketch. 



" These different appearances were reobserved, in all their curious dis- 

 Bolving-view-hke changes, several times over, thus beautifully showing the 

 harmony that may possibly exist between the most dissimilar observations, 

 and strongly suggestive (to my own mind) that the form of the shading on 

 the floor is permanent, and that the various degrees of visibility of its more 

 difiicult features is owing to the relative changes in the medium through 

 which we see it — whether of the earth's atmosphere alone, or of an obscuring 

 medium on the floor itself, must be determined by comparison with similar 

 and simidtaneous observations in different parts of the world. Thus within 

 two hours I several times saw four greatly differing aspects of the shading on 

 the floor, viz. B. & M.'s, Mr. Birt's, Mr. Knott's, and my own." 



1869, August 21. — Mr. Gledhill recorded the tint of floor as dark as that 

 of the Mare Crisium, and that the light sector was fairly seen. 



Fig. 6, August 23, 1869. — We find in this figure the extension and 

 gathering up of the dark portion still in progress, although to a very small 

 extent as compared with the "action" of the 16th to 17th. The northern 

 light portion was seen separated from the N.W. arm of the trident, and an 

 opening made from spot No. 6 to the S.E, border, apparently by the action 

 of the spot. Some additional spots were seen on the 23rd — viz. No. 5 on the 

 west border of the S.E. arm of the trident, No. 22 on the N.W. arm, No. 16 

 on the northern opening, and No. 7 near the east border. 



