86 Mr. Mossotti on the Constitution of the 
be formed by cutting out the internal portion of a disc, and 
the sun is situated near its internal edge. 
I would now entreat you, Gentlemen, to lend me your at- 
tention while 1 proceed to examine whether this form of ne- 
bula agrees with the mechanical conditions of a permanent pre- 
servation. According to the beautiful theory which the ce- 
lebrated Laplace has given of the attraction of Saturn’s ring, 
and according to the doctrine which Professor Plana has set 
forth in the 24th volume of the Memorie dell’ Academia di 
Torino, on the attraction of bodies of various shapes, we 
conclude that if a body is situated near either the interior or 
the exterior edge of a plane ring, the body is attracted towards 
a point situated nearly in the centre of the thickness of the 
ring. If then we consider the resultant of the innumerable 
attractions of the stars distributed in the annular space of the 
Milky Way as the same with that of a continuous ring, it 
follows that a star placed in the internal edge of the Milky 
Way will tend to penetrate into the adjacent annular space, 
near the centre of which it would find its position of equi- 
librium. On reaching this central point, however, with the 
velocity generated in its motion, it will not stop there, but 
will proceed by the law of inertia towards the external edge ; 
as soon as the velocity of the star is destroyed by the force of 
attraction, which acts now in the opposite direction to that of 
its motion, it will change its path, and will again pass through 
the centre of the annular space with a certain velocity, and 
will proceed towards the internal edge, to the place whence it 
first started. Having returned here, it will tend to repeat 
another similar oscillatory motion, then a third, and will thus 
go successively oscillating between the two edges of the plane 
of the annular space. 
Let us suppose now that the star is projected with a certain 
velocity in the direction of the tangent at the point of the 
curve in which it is situated ; and let us consider the resultant 
motion arising from the motion thus impressed and from the 
oscillatory motion above described. According to the princi- 
ples of mechanics, it appears that the velocity of projection may 
be suchas to make the star move repeatedly round over the 
been the case ; but Dante’s verses must not be taken in an absolutely literal 
sense. The constellation of the Cross has about 30° south polar distance ; 
it is visible and rises several degrees above the horizon in Hindostan and 
in Arabia, as for instance at Mecca: and the existence of the principal 
stars which form it, although commonly unknown, could not beso entirely, 
and especially to such as were acquainted with astronomy, as Dante was, 
In fact, the stars of which this poet speaks, and of which Royer in 1679 
formed the constellation of the Cross, are found even in Ptolemy’s Cata- 
logue, though not exactly determined in position. 
