1875.] Selenography : its Past, Present, and Future. 205 
measures of each spot made, the positions of all except 
8 were only approximate, the average error being about one 
degree. Pressure of other engagements, especially his lunar 
tables, and his early death in 1762, prevented his carrying 
out his intentions, but in 1775, amongst his ‘‘ Opera 
Inedita,”” was published a small map 8 inches in diameter, 
founded on his measures, which remained for many years the 
best lunar map, and was employed by Schroter. 
From a consideration of careful micrometrical measures 
of the position of Manilius on three occasions, in October, 
1763, Lalande obtained for the value of the inclination of 
the moon’s equator to the ecliptic 1° 43’, which he regarded 
as exact, though founded on so few measures; whilst he 
regarded his results as showing the coincidence between the 
nodes of the lunar equator and orbit, as demanded by 
Cassini’s theory of the moon’s librations. 
From its interest and importance the question was 
attacked theoretically by Lagrange, in an important memoir 
in 1764, where, by an analytical investigation, it was shown 
that the equality of the periods of the lunar rotation and 
orbital revolution was a necessary consequence of the 
earth’s attraction; and further pointing out that the earth’s 
influence was capable of producing this equality, even had it 
not originally existed. Later, in 1780, Lagrange showed 
that the coincidence between the nodes of the moon’s 
equator and orbit involved in Cassini’s theory of its libration 
was a necessary sequence of the earth’s attraction. It 
also was pointed out by Lagrange that the moon’s form must 
be that of an ellipsoid whose shortest axis was its polar 
axis and longest axis directed towards the earth’s centre. 
During the period 1784—1802, Schroter, of Lienthal, in 
Hanover, with powerful reflecting telescopes, made many 
observations of the moon’s surface, which he was the first to 
examine in detail, and he obtained what he believed to be 
proof of the existence of a lunar atmosphere and of active 
volcanic changes. Though many of Schroter’s observations 
were important, and his drawings, though rough and 
wanting in detail, are faithful, he took insufficient precau- 
tions against changes produced in the appearance of lunar 
objects by variation of libration and illumination, which 
were, indeed, then little understood ; consequently, many 
of his supposed changes and evidences of an atmosphere 
have been shown to be fallacious, and have involved the 
rest in perhaps undeserved discredit. The results of 
Schroter’s observations were published in his ‘‘ Selenotopo- 
graphische Fragmente,” the first volume appearing in 1791 
and the second in 1802. 
