88 Mr. Mossotti on the Constitution of the 
the Milky Way may therefore constitute an unchangeable 
system, circulating in an annular space to which they are al- 
ways limited, but to the internal or external circumference of 
which they will approach alternately in their motion. 
The interesting result lately published by Mr. Argelander 
of his researches on the direction of the motion of the sun, af- 
fords a confirmation of the possibility that the motion of that 
luminary and of his planets should be such as we have de- 
scribed it. The first attempt to discover the sun’s motion is also 
due to Sir W. Herschel. When we reflect on the immensity 
of the dimensions of the orbits, on the length of the periods 
required for completing the revolutions of the stars; when we 
consider that the circular movement of the ring is probably 
in the same direction with respect to all the stars, and that it 
is only their relative motion that we can more immediately 
perceive, it becomes evident that a long course of centuries 1s 
required to manifest the change of position of our solar system ; 
_ notwithstanding this, Sir W. Herschel, ever since 1783, had 
indicated the star A of the constellation Hercules as the point 
to which the sun tends in his motion. Against this fact many ob- 
jections had been raised which rendered it less positive: but the 
uncertainty which still remained was finally cleared up by the 
above-named distinguished astronomer, late Director of the 
Observatory at Abo. For having subjected this question of 
sidereal astronomy to a more accurate calculation, by the dis- 
cussion of an increased number of observations, he discovered 
that our solar system is moving towards a point of the con- 
stellation Hercules near the star 143 of the 17th hour accord- 
ing to Piazzi’s Catalogue, which point differs little from the 
one noted by Herschel. ‘The sun being at present, as we ob- 
served, in a point of the internal edge of the Milky Way, just 
at the moment of ceasing to approach to it, or rather in the 
act of receding from it, must in this deflection of its path haye 
a direction of motion nearly in the tangent to the curve which 
forms the internal limit of the annular space at that point. 
Now, by a remarkable coincidence, if from the constellation 
of the Cross we draw a tangent to the circle of the Milky Way 
in a direction slightly elevated towards the northern hemi- 
sphere, we find that it must pass near the above points of the 
constellation Hercules. The solar system revolves, there- 
fore, in the Milky Way round the centre of gravity of this 
mass of stars; and with an analogy which we must not omit 
to notice, proceeds round this centre from west to east, exactly 
in the direction in which all the bodies of this system revolve, 
the greater round the lesser. 
To give in a few words a clear image of what has been said, 
consider a cluster of countless stars in the immensity of space, 
