OF THE COSMOS. PERIODICALLY VARIABLE STARS. 167 



star also shews nearly the same irregularities as Mira : 

 the deviations of the observed maxima from the results 

 calculated on the supposition of a uniform period are as 

 rrluch as 40 days, but are very greatly diminished by the 

 introduction of a perturbation of 8 J single periods, and 

 another of 100 such periods. At its maximum, of 

 brightness, this star reaches, on the mean, a faint 5m., or 

 one gradation brighter than the star 17 Cygni. But 

 here also the fluctuations are very considerable, and have 

 been observed from 13 gradations below the mean to 10 

 above it. If the star never exceeded the weaker maximum, 

 it would be altogether invisible to the naked eye, whereas, 

 in 1847, it could be seen for fully 97 days without a 

 telescope ; the mean duration of its visibility is 52 days, 

 of which 20 days in the mean are occupied by the increase, 

 and 32 by the decrease. 



4. 30 Hydra Hevelii, E. A. 200 23' ; Decl. 22 30'. 

 Of this star, which from its position in the heavens can 

 only be seen for a short time in each year, all that can yet 

 be said is, that both its period and its maximum brightness 

 are subject to great irregularities. 



5. Leonis E=420 Mayeri; E. A. 144 52', Ded.+ 

 12 1'. This star has often been confounded with the 

 neighbouring stars 18 and 19 Leonis, and has on that 

 account been very little observed ; it has, however, been 

 sufficiently so, to shew that the period is rather irregular. 

 The maximum brightness also appears to fluctuate through 

 some gradations. 



6. n Aquilae, also called 77 Antinoi; E. A. 296 12'; 

 Decl-f 37'. The period of this star is tolerably 

 uniform, 7d. 4h. 53m. 53s.; but yet the observations 



