56 
This was followed by some considerations of lunar forces and pro- 
babilities of a chronological sequence to be detected amidst the over- 
crowded evidences of a wonderful activity. Our own ablest selenologist, 
_ Mr Birt, occupied the place of honour in this work. The Rev. T. W. 
Webb had done much in this direction also. Their opinions, as to the _ 
order of past activity, were mentioned, and several examples illustrat- 
ing them were pointed out. 
Then, the order of activity perceived, another question suggested 
itself. Had all activity ceased? or was it possible that it might be 
working still? Inexplicable differences between observations made at 
different times and places were adduced, and the recent case of Linné 
referred to, showing that past records were at variance with the present 
aspect of the object and proving, at the same time, that the maps and 
drawings of the older observers were not sufficiently accurate for - 
modern purposes. 
Mr. Pratt then explained that more than three years since, he 
determined to test the questions of a Lunar atmosphere and present 
activity, as far as his means would allow, and for this purpose selected 
some regions for study. As it seemed probable that the greatest op- 
portunity of detecting change or activity would be in the very minutest 
of objects,—those but just included within the limits of vision,— his 
plan was to map these in certain regions, so as to produce standards 
of reference, and then to continue an unremitting watch on all pos- 
sible opportunities, so as to test their stability or otherwise. He dis- 
claimed all bias to either side at the commencement of his observations, 
and owned a complete indifference to their results, although to ad- 
vance what was true in the matter was at once his object and ambi- 
tion. One of his selections was the walled plain Plato, on the margin 
of the Mare Imbrium. Beer and Madler, in their celebrated map, had 
- only placed four little craterlets on its floor, and reported the presence 
of some light streaks. Mr. Webb had thrown doubt on their arrange- 
ment of the latter, and the craterlets had been scrutinised without much 
addition to their number. However, Mr. Pratt’s observations quickly pro- 
ved productive, and in February, 1869, he forwarded to Mr. Birta plan 
of the floor, showing more than double the number of previously 
known objects. The work had since attained much importance in 
consequence of the intense interest taken in it by Mr. Birt, and the 
yaluable co-operation of other observers which he secured. During 
the three years more than two thousand observations of the minute 
