90 Mr. Mossotti on the Constitution of the 
II. As the Cavalier Carlini, the renowned astronomer of the 
Observatory at Milan, has pointed out to me some parts of the 
foregoing discourse, which, unless more fully developed, seem to 
leave some difficulties in the hypothesis of the sidereal system set 
forth in it, I avail myself of this opportunity to explain them. 
The first point is the modification, which, without any rea- 
son being adduced, I have apparently given to the ideas set 
forth by Sir J. F. W. Herschel, by placing the sun on the in- 
ternal edge of the ring, whereas the passage I have quoted 
seems to allude to the sun as being placed excentrically within 
the ring rather than as forming a constituent part of it. It 
is however easy to see that the distinguished philosopher in 
those few lines wished merely to throw a luminous idea on 
the cause which might produce the appearance of a difference 
in the brightness of the two portions of the Milky Way, 
without having any other object in view. When we bring 
into consideration also the circumstances of the motion pro- 
duced by a mutual attraction, it is evident that all the bodies 
in the interior of the ring are brought to form a part of the 
ring, and to cross it from time to time, and this must be the 
case with our sun also. It is probably in moving across the 
ring that the temperature of the solar system is greatly in- 
creased, according to the plausible hypothesis by which M. 
Poisson explains the internal heat of the terrestrial globe, and 
the geological changes to which it has been subjected. Some 
stars will depart more, others less from either side of the czr- 
cumference of equilibrium, according to the places in which they 
were situated, and according to the direction and the velocity 
with which they were put in motion. Our sun is probably one 
of those that depart furthest from it, and descend further into 
the empty space within the ring. 
The annular-shaped system of stars of which we are 
speaking, is not singular in the heavens. Other nebulae have 
been observed of this shape, and among others one very distinct 
situated between the stars 8 and y Lyree, and presenting in its 
appearances the very peculiarities which we have described in 
our system. 
The second point is the circumstance, that if we are placed 
on the interior limit of the ring on the side where the constel- 
lation of the Cross is situated, the Milky Way ought to ap- 
pear brighter, not in the direction of this constellation, but 
rather in the lateral directions, since in these directions the 
visual rays cross a greater extent of the thickness of the ring. 
But if attentively considered, this circumstance, instead of 
throwing any difficulty in the way, affords a new and not slight 
argument in favour of the assumed hypothesis. 
