90 . REPORT — 1871. 



in the 8ame proportion, indeed not so high as on some nights when fewer 

 spots have heen visible. The supposition of Schroter of an exceedingly low 

 atmosphere, confined to the lower regions, seems to me especially consonant 

 with the above observations, for the following among other reasons : — 



" A thin atmosphere, the only possible detection of which is confined to the 

 lower parts of the floor [that is within the mountainous enclosure of Plato], 

 may obscure the streaks partially [to effect this there must be condensed 

 material of some kind] without affecting the spots, which, if craterlets, are 

 raised more or less above the level of the streaks [the low fogs, the upper sur- 

 faces of which are at a less elevation than ordinary buildings are high, may be 

 cited as examples] ; for such an atmosphere would probably be rendered more 

 dense by and during the supposed activity in the spots, which on that night 

 were unusually bright and, according to the hypothesis, in action. [It must 

 not be forgotten that on comparing the observations of Mr. Pratt with those 

 of Mr. Gledhill, the presumption is that the iinusual number and brilliancy 

 of the spots was simply the effect of a finer atmosphere at Brighton as 

 compared with that at Halifax. The phenomenon which is at variance with 

 this is the less brihiancy of the streaks as recorded by Mr. Pratt ; still we 

 have the bright streaks of Mr. Gledhill supporting the hypothesis of the effects 

 of the earth's atmosphere.] Hence after a subsidence of the brightness of the 

 spots and the restoration of the normal state of the atmosphere, we might 

 expect to see the streaks come out more distinctly." 



It wiU be remarked that, in my suggestions above, the increased bright- 

 ness of the streaks is supposed to depend upon the craterlets actually 

 ejecting material, while the increased brightness of the spots depends upon 

 the escape of vapour. I have not quoted Mr. Pratt's remarks for the 

 purpose of controverting them ; they appear to me to be exceedingly 

 valuable, and in the present state of selenological inquiry it is important 

 to canvass every view that may be put forward. It is quite consonant with 

 both our views that increased activity in a spot may, and doubtless does, 

 manifest itself by increased brilliancy ; and it is not unlikely that the forma- 

 tion of a spot in the way suggested over a volcanic orifice otherwise invisible 

 may precede an actual eruption, contributing to an increased brilliancy of 

 the streaks if they really result from volcanic ejecta. 



On the agencies capable of affecting the visibility of objects on the moon 

 Mr. Pratt remarks : — " To my own mind the only likely agencies that can 

 exist in the moon capable of affecting the visibility of objects are the every- 

 where-denied lunar atmosphere and real volcanic activity ; as far as I can 

 learn, the observations of some favour the one agency, while other observations 

 do the same for the other, at the same time that different observei-s 

 alternately deny the possible existence of either. Surely they are very 

 closely related. If volcanic activity be established, can it exist without 

 an atmosijhere ? AVhile if a low atmosphere be established, would not the 

 stronger objection to present volcanic activity be removed? The hope that 

 persistent and minute observation of a suitable region might produce a 

 result sufficient either to weaken or strengthen the supposition has been at 

 once the impetus and bond which has induced me to give a large share of 

 attention to Plato. "We may not have attained such a result even yet ; but 

 possibly continued application may be rewarded. I hope so. The close 

 study of typical species is generally the best method of acquiring a good 

 knowledge of genera." 



Mr. Pratt further adds: — " The reverse of what I have here stated I have 

 several times observed, viz. that the light streaks on those occasions were 



