364 Mr. T. S. Aldis on Goodricke's Theory of Algol. 



Algol ; so such a medium would not retard the satellite, but 

 move with it. The medium can be none other than the inter- 

 stellar medium, through which the stars pass from point to point 

 without impressing on any part any sensible rotation. It fol- 

 lows naturally, then, from Goodricke's theory, that the period 

 should be diminished, as the light medium must produce some 

 effect on so rare and rapid a body. 



Yet the diminution of period is very small. It amounts to 

 about one second in twelve years at most. This gives in one 

 revolution a diminution of about onnn \ nnnn of the whole. Now 



3600 0000 



for Encke's comet there is a diminution of ■! of the period 

 in one revolution. Of course the circumstances are very different; 

 but allowance being roughly made for them, the comet appears 

 to suffer most. This is probably due partly to the less density 

 of the comet, partly to the fact that the light medium is not the 

 only resister of the comet. Whatever interplanetary media there 

 may be rotating round the sun, the comet in its perihelion, 

 moving far faster than they, will be hindered, whilst the con- 

 trary action at aphelion would be far from sufficient to restore 

 the balance. Tidal lagging, too, or, without lagging, the con- 

 traction of Algol and its attendant's spheroidal forms as they 

 cool, would tend to diminish the mean motion, and so prevent 

 its increase being as great as it otherwise would be. It thus 

 appears probable that the diminution of Algol's period may be 

 accounted for by Goodricke's hypothesis. This diminution will 

 continue till, in the process of condensation, tidal lagging comes 

 into greater play, when it may be slowly reversed. 



The above calculations are rude and approximate. It would 

 take more time than the question is worth, even if we had suffi- 

 cient data, to discuss the problem thoroughly. I shall be glad 

 if this paper be found to advance a step in our knowledge of the 

 heavens. 



Note. — In the paper " On the Nebular Hypothesis/' October 

 1869, it may be noticed that the elongation of form, which 

 theory suggests for the attended planets as their condition on 

 severance from the central mass, is probably none other than the 

 crescent form, like a fragment of a ring partially embracing the 

 central body, which is by no means without example in the va- 

 rious nebulae. 



Manchester Free Grammar School, 

 April 14, 1870. 



