36 
Variable Stars . 
[January, 
approximately; consequently our planet must make one 
complete revolution in that time. Wherefore, by our sup- 
position as to mass and volume, a planet to revolve once in 
6g‘83 hours must travel at a mean distance from Algol’s 
centre of about 3,670,000 miles. But Algol undergoes a 
more or less diminution of light for nearly eight hours ; 
therefore our planet, when in transit, must travel over a 
space equal to the diameter of Algol in that time. It fol- 
lows, then, that at the time the planet is travelling with a 
velocity of 30 miles a second ; consequently its orbit cannot 
be nearly circular, for so as to travel in a circular orbit at a 
distance of 3,670,000 miles would require a velocity of 
91 miles per second, or fully three times the observed 
amount ! Then, because its velocity when in transit falls 
short of the velocity necessary for a circular orbit, its orbit 
must be an ellipse with Algol in one of the foci. Fig. 1 
represents an orbit which would do. In this figure the lines 
parallel to ax represent the direction in space of the solar 
system ; the planet travelling along b b' when in transit at a 
distance from Algol’s centre of about 6,500,000 miles, passing 
at each return close to the latter’s surface. And since the 
mean distance a m is 3,670,000 miles, and a l about 450,000 
miles, the planet at its greatest distance must reach to nearly 
6,900,000 miles from the ruling focus. 
We see, then, that given the above data it is a compara- 
tively simple matter to deduce a theoretical planet whose 
motions will accord with the fluctuations in the light of its 
primary. But we have omitted one element which creates 
a serious difficulty. It is this : the maximum light of Algol 
bears to its minimum the proportion of something like 
5 to 1 ; and this is a consideration which seems to me to 
completely negative the satellite theory. 
Let us pause to inquire into the conditions of this new 
feature in the problem. 
