1885.] 
Variable Stars. 
39 
of a vastly remote possibility. If we assume, for argument’s 
sake, that the Sun, when he has parted with his heat, will 
have a mean density equal to the Earth’s (a very moderate 
estimate, since it will probably exceed it many times), its 
mass naturally remaining the same, we shall see that his 
volume will have to contract into a space occupying only 
one-fourth of that which he now occupies. Consequently if 
the satellite of Algol equals the Earth in density (as we have 
seen, a very low estimate considering his size), and Algol 
itself in density equals the Sun, we arrive at the happy result 
that the centre of the heat, life, and light of the system of 
/ 3 Persei is a secondary body in its own system, — since its 
mass must be much less than that of the opaque orb, — a 
state of things highly adapted to the philosophy of the 
Ptolemaists. 
If it were worth while, it would be easy to deduce a cir- 
cular, instead of an elliptic, orbit for the system of Algol, 
with mass and diameter to match. Such an inquiry, how- 
ever, would be without value, since it would leave absolutely 
untouched the arguments urged against the third postulation. 
Whatever the size or shape of the orbit, or whatever the 
size of Algol, the opaque body would still bear the same 
proportion, namely as 8 is to 9, to Algol itself. 
Another celebrated variable is Mira, of the constellation 
of Cetus. This star passes through its cycle of changes in 
a little over 331^ days. Unlike Algol, which is normally at 
its brighest, Mira is only at its maximum of brilliancy during 
a period which bears a very small ratio to its period of 
minimum. Furthermore, its variability is not so regular as 
that of Algol. Its greatest brilliancy is not always of equal 
intensity; but, at the same time, it is supposed that the 
different effects occur with periodical regularity, and are 
only minor variations of the great cycle of changes. 
Manifestly none of the above theories explain the changes 
of this star. Even the satellite theory, the most probable 
of them all, would require, it would seem, bodies whose 
motions should be altogether independent of the laws of 
gravitation. Any pianet, or number of planets, to effedt the 
observed changes, would have to rush hither and thither at 
times, in a highly erratic manner, to the not small danger 
of the whole system the while ; but settling down calmly, 
for about five months, in such a position as to cut off 
nearly every particle of the light which Mira sends to the 
Earth. 
We see, then, that every theory yet propounded is de- 
fective, insomuch that, although they may be successfully 
