BY JOHN TEBBUTT, JUN. 129 



observation of this star by Argelander, Heis, and Schmidt, its 

 period is found to be shorter now than when it was first dis- 

 covered. 



The period is found to diminish, not progressively, but with 

 accelerated rapidity. It is impossible to foresee what the final 

 result of this gradual diminution will be. The star may even- 

 tually become of constant lustre, or its period, after having arrived 

 at a certain state, may again lengthen with accelerated rapidity. 

 The law and cycles of its variations have, of course, to be deter- 

 mined from a long course of future observation. There are also 

 great apparent irregularities attending the variations of Omicron 

 (o) Ceti, another remarkable star in the class we are considering. 

 The maximum of this star for 1862, according to Pogson's 

 Ephemeris, occurred on July 3rd, its magnitude then being the 

 second ; it is well known that at its minimum it becomes invisible 

 to the naked eye. It is a star which commends itself to the 

 close attention of Southern observers. Although it is within 

 reach of European observers, still the conditions under which 

 it is seen by most of them are not so favourable as those under 

 which it is seen in this latitude. Few Northern observers have 

 the advantage of clear skies like those of Australia. No observa' 

 tions, perhaps, are more difficult of accurate performance than 

 those attending variable stars, both on account of the many 

 sources of error to which they are liable, and because so much 

 depends on the observer's judgment. The more numerous, then? 

 the observers of a particular star, the greater reliance is to be 

 placed on the general result. Every variable star should, if 

 possible, be observed about the time of its superior meridian 

 transit, as atmospheric causes interfere much with the accuracy 

 of the results. I should be trespassing beyond the limits of a 

 paper were I to enter into a description of all the known variable 

 stars ; it will be sufficient to give a table of those which are ad- 

 vantageously situated for observation in this hemisphere. Such 

 a table will be found appended to this paper. But after having 

 spoken of Algol aud Mira Ceti, I must not forget to say a few 

 words with reference to our remarkable Southern star Eta (77) 

 Argus, which more nearly concerns us in this latitude. This 

 star has, for the last thirty-five years, been known to be variable. 



