1875.)  Selenography: its Past, Present, and Future. 203 
accuracy of his map shows the great care and success with 
which his observations were made, and for many years it 
remained the best chart of the moon existing. As is well 
known, he adopted as a method of nomenclature, a system 
based on the fancied resemblance between the lunar and 
terrestrial formations; whilst from estimating the distance 
within the dark portion of the moon, that the summit of the 
peaks remained visible, he computed that some of the lunar 
mountains must be 17,000 feet high—a very core¢t estimate 
of probably the Apennines, the mountains measured. 
Four years after the publication of the “ Selenographa f 
of Hevelius, in 1647, appeared the “ Almagest” of Riccioli, 
of Bologna, containing a lunar map, less perfect ona whole 
than that of Hevelius, ” but still a very creditable result; and 
in it was introduced a much improved system of nomen- 
clature, where the names of distinguished astronomers and 
mathematicians were introduced in place of the feeble ter- 
restial analogies of Hevelius. On the Continent this new 
system met with general acceptance, and has, since the date 
of Schroter’s observation, entirely superseded the earlier, 
though, until the close of the last century, it was still 
employed in England. 
Newton turned his attention to the subject of the lunar 
librations, and in a letter to Mersenius, in 1675, amended 
Hevelius’s explanation of the cause of the lunar libration in 
longitude, by showing it was a necessary sequence of the 
uniform rotation of the moon on its axis, in the same time 
as a complete revolution in its orbit, combined with the 
variable motion of the moon in its orbit ; but, like Hevelius, 
considered the axis of rotation to be perpendicular to the 
plane of the ecliptic. Later, Newton also showed that, as 
a consequence of the earth’s attraction and the coincidence 
of its period of revolution and rotation, the moon must be 
elongated in a direction passing through the centre of the 
earth when in mean libration, which was the cause of the 
moon always presenting the same hemisphere to the earth. 
From this spheroidal form of the moon, Newton pointed out 
a real libration of the moon must ensue from the retarda- 
tion exerted by the earth’s attraction to the longer axis 
of the spheroid, passing from a line directed to the earth’s 
centre, though compelled to do so by the libration in 
longitude, causing thus a vibratory motion in the direction 
of this longer axis. 
Selenography received its next advance from the great 
astronomer Dominic Cassini, who, by investigating the 
known conditions affetting the lunar observations, and 
