1875.) Selenography : its Past, Present, and Future. 207 
was in them, thus affording a complete confirmation of the 
theory of libration first announced by Cassini, and shown to 
be theoretically correct by Lagrange, Laplace, and Poisson. 
From these observations Nicollet also deduced that there 
existed a real libration of the moon in longitude corre- 
sponding to the annual equation in the lunar theory, and 
amounting to 4’ 46” of selenographical longitude. 
With a view of further improving these interesting and 
important results, Nicollet made a series of 32 new mea- 
sures of the position of Manilius during 1819-20, and 
combining these with the 124 measures formerly employed, 
and the 18 made by Arago and Bouvard in 1806, he deter- 
mined that the maximum real libration in selenographical 
longitude corresponding to the annual equation in the lunar 
theory amounted to 4’ 49°7”, and the figure of the moon, as 
deduced from this value, Nicollet showed was inconsistent 
with the figure of equilibrium it would have possessed had 
it been originally fluid. Poisson pointed out, however, 
that the data was insufficient for the rigid determination of 
this point, and also computed, from Nicollet’s results, that 
the real libration in longitude corresponding to the moon’s 
ecliptic inequality, only amounted at a maximum to 39”, or 
less than one-fifth of a second of arc. 
From the same observations Nicollet deduced the value 
1° 28’ 45" for the inclination of the lunar equator to the 
ecliptic, and found for the selenographical position of the 
centre of Manilius latitude 14° 26’ 54”, longitude 8° 46’ 56”. 
These results were in the highest degree interesting, and 
possessed no small importance, but, as remarked by Nicollet, 
Bouvard’s optical means were very small, and it was very 
desirable that the investigation should be repeated not only 
with far more powerful instruments, but a greater number 
of observations be employed. 
For the really systematic observation of the lunar surface, 
with the view of the solution of the many interesting and 
important questions connected with it, the accurate deter- 
mination of the position of all the principal formations was 
essential, as by no other means could the identity of the 
various objects observed at different times be ascertained, 
and any changes that might occur be recognised. Hitherto 
there had, however, existed no data on which to found a 
trustworthy map of the principal features of the lunar sur- 
face ; for with the exception of some eight formations whose 
position had been fixed by Mayer, and the very accurate 
determination by Nicollet and Bouvard of the place of 
Manilius—one of them—all the other formations had been 
VOL. V. (N.S.) 2D 
