256 REPORT— 1872. 



ducing the lower lighter surface ? The difficulty in this supposition is the 

 dark upper surfaces of the hypothetical clouds. If we can find a vapour 

 which, when illuminated by the sun, appears darker than the ordinary sur- 

 face of a cloud of condensed aqueous vapour in sunlight (which, by the way, 

 shines with about the same luminous intensity as the moon's surface), we 

 shall be in a fair way of explaining the difficulty. The green colour which 

 is witnessed at sunrise and sunset is probably the natural colour of the floor 

 of Plato, which, under the accumulated heat of the solar rays, reflects a 

 lighter tint ; at the same time the condensed vapour overspreads the lighter 

 floor below, giving rise to the appearances we witness. 



In applying this hypothesis to the explanation of the phenomena presented 

 by Linne, it is necessary to remark that we have a shallow basin surround- 

 ing a cone. The earliest appearance is that of the cone standing out from 

 the dark surface around, the next of a white spot, more or less of a cloudy 

 character, surrounding and hiding the cone. If the above-enunciated hypo- 

 thesis be correct, it would explain the Linne phenomena thus — at sunrise 

 the features are well marked, as neither a liberation of gas nor its condensa- 

 tion has taken place : as the sun's altitude increases, the surface around 

 Linne becomes heated, reflects more light, and a spot is formed at the same 

 time in the lower levels ; the condensation of liberated vapour not far above 

 the surface produces the generally observed dark appearance of the Mare, 

 obscuring on many occasions spots and craters. The remarkable instance 

 of the contraction of the white spot around Linne in June 18G7 may be 

 explained by the condensed vapour rising higher, so as to leave a smaller 

 area of the upper part of the cone visible ; or if it were at an elevation equal 

 to about the summit of the cone of Linne, a partial cessation of the liberation 

 of elastic vapour would allow it to gather around the cone, to be dispersed 

 by a further eruption of vapour or gas from the orifice of the cone. 



On the 22nd of September, 1871, I received a letter from Mr. Elger, in 

 which he says : — "Your letter of the 18th, relating to the markings on the 

 floor of Plato, has greatly interested me, inasmuch as you therein suggest a 

 theory to account for the remarkable appearances observed, which seems to 

 me to be well worth careful consideration. Doubtless the sun's heat during 

 the long lunar day must cause vaporization on the moon's surface, and sub- 

 sequent condensation of the various vapours raised follows as a matter of 

 course. So far we have a vera causa which would account for a great deal, 

 if we could only show that the vaj)ours raised are sufficiently dense to produce 

 VISIBLE effects : it is highly probable that they are so, or, at all events, we 

 may suppose that the varying visibility of such delicate objects as the spots 

 on the floor of Plato is, to a certain extent, accounted for in this way ; but I 

 would submit that, in the case of the markings, it is difficult (if the dark 

 spaces between the markings are the upper surfaces of masses of vapour 

 hovering over the floor, and of course varying in altitude from hour to hour) 

 to account for the fact, that since the year 1866 the light streaks have 

 altered so little in shape and position. It seems to me that if they were 

 merely openings in the dense vapour, they would not only vary in position 

 from lunation to lunation, but changes would take place from hour to hour, 

 which could hardly be overlooked by observers with powerful instruments : 

 Linne is a case in point. The observation of June 1867 is in perfect har- 

 mony with your theory ; indeed the phenomena presented by this remark- 

 able formation are, I think, quite inexplicable, except by supposing agencies 

 of the kind suggested by j^ou to be in operation. At some future time, 

 when selenography is more advanced, it will probably be found that all 



