1875.]  Selenography: its Past, Present, and Future. 22 
inadequate optical means employed by Bouvard ; and Nicollet 
considered it advisable that the whole research should be 
repeated with the assistance of more powerful instruments. 
On these grounds Wichmann, in 1847, made a fresh attempt 
to determine the amount of real libration of the moon from 
a series of 44 independent observations, but entirely failed 
to obtain a satisfactory determination of the amount of real 
libration, whilst the discrepancies between the results of 
the two memoirs is additional reason for further inves- 
tigation. 
The results obtained renders a new investigation of this 
subject very desirable, for it is only by this means that the 
limb of the moon can be rendered a satisfactory origin of 
measures for determining the position of the standard points 
on the surface of the moon, to be afterwards employed to 
measure other lunar objects from; and it is only the 
limb that is available for this purpose, whilst the 
sole standard points already existing are the central moun- 
tain of Manilius and the small lunar crater Mosting A. 
Moreover, the disadvantages of being obliged to empioy the 
lunar limb as an object to be measured, or to measure from, 
has long been recognised in astronomical work, apart from 
selenography, where exactness is necessary, as in the deter- 
mination of accurate longitude from moon culminations. 
For such purposes it has long been proposed to employ, 
instead of the somewhat indefinite limb, with its variable 
irradiation and irregularities, a bright lunar crater, without 
these disadvantages; but this is inadmissible with the 
moon’s real libration, still only indefinitely known. 
In repeating, however, the research on the real librations 
of the moon, several modifications might be adopted, besides 
simplifying the preliminary reductions employed by Nicollet 
and Wichmann, and it might be made after the following 
plan. Madler has already pointed out that a small brilliant 
crater is preferable as a point to be employed to measure to 
any central mountain, like that of Manilius, used by Bouvard 
and Nicollet, and has suggested Mosting A and Triesnecker 
C and Bas suitable both in form and from lying close to 
the centre of the moon, a desirable feature ; and the former 
consequently, at the instance of Bessel, was employed 
by Wichmann. Of these, Mosting A is alone well suited for 
this purpose, but a still better is almost the crater Tries- 
necker A; whilst-a third can be selected from one of the 
three craters, Hipparchus C, G, or E, the last being most 
favourably placed, and all considered perhaps the most. 
suitable for this object. Three points have been selected ; for 
