VARIABLE STARS. 175 



«ne gradat/on brighter than the star 17 Cygiii. The Huctaations, how 

 ever, are in this case also veiy considerable, and have been observed 

 from thirteen gi-adations below the mean to ten above it. At this low 

 est maximum the star would be perfectly invisible to the naked eye, 

 whereas, on the conti-ary, in the year 1847, it could be seen without 

 the aid of a telescope for fully ninety-seven days ; its mean visibility 

 extends to tifty-t^vo days, of which, on the mean, it is twenty days on 

 the increase, and ^irty-two on the decrease. 



(4) 30 Hydr® Hevelii, R. A. 200° 23', Decl. —22° 30'. Of this star, 

 which, fi'om its position in the heavens, is only visible for a short time 

 during every year, all that can be said is, that both its period and ita 

 maximum brightness are subject to very srreat irregidarities. 



(5) Leonis R. =420 Mayeri ; R. A. 144'^ 52', Decl. +12^ 7'. This 

 star is often confounded with 18 and 19 Leonis, which are close to it, 

 and, in consequence, has been very little observed ; sufficiently, how 

 ever, to show that the period is somewhat irregular. Its brightness at 

 the maximum seems also to fluctuate through some gradations! 



(6) 71 Aquilas, called also 77 Antinoi ; R. A. 296«^ 1^2', Decl. -j-QO 37'. 

 The period of this stai* is tolerably uniform, 7d. 4h. 13m. 53s. ; observa- 

 tions, however, prove that at long intervals of time trifling fluctuations 

 occur in it, not amounting to moi'e than 20 seconds. The variation of 

 light proceeds so regularly, that up to the present time no deviations 

 have been discovered which could not be accounted for by errors of ob- 

 servation. In its minimum, this star is one gradation fainter than i 

 Aquilae ; at first it increases slowly, then more rapidly, and afterward 

 again more slowly ; and in 2d. 9h. from its minimum, attains to its great- 

 est brightness, in which it is nearly three gradations brighter than /?, 

 but two fainter than 6 Aquil.B. From the maximum its brightness does 

 not diminish quite so regularly; for when the star has reached the bright- 

 ness of /3 {i. e., in Id. lOh. alter the maximum), it changes more slowly 

 than either before or afterward. 



(7) /? Lyra?, R. A. 281° 8', Decl. -f330 11'; a star remarkable from 

 the fact of its having two maxima and two minima. "When it has been 

 at its faintest light, one third of a gradation fainter than f Lyrse, it rises 

 in 3d. 5h. to its first maximum, in which it remains three fourths of a 

 gradation fainter than y Lyrte. It then sinks in 3d. 3h. to its second 

 minimum, in which its light is about five gradations greater than that of 

 ^. After 3d. 2h. more, it again reaches, in its second maximum, to the 

 brightness of the first ; and afterward, in 3d. 12h., declines once more 

 to its greatest faintness; so that in 12d. 21h. 46m. 40s. it runs through 

 all its variations of light. This duration of the period, however, only 

 applies to the years 1840 to 1844; previously it had been shorter — in 

 the year 1784, by about 2ih ; in 1817 and 1818, by more than an hour; 

 and at present, a shortening of it is again clearly perceptible. There 

 is, thei'efore, no doubt that in the case of this star the disturbance of ita 

 period may be expressed by a foniiula of sines. 



(8) 6 Cephei, R. A. 335° 54', Decl. -f 57° 39'. Of all the known va 

 riable stars, this exhibits in every respect the greatest regularity. Th« 

 period of 5d. 8h. 47m. 39is. is given by all the observations from 1784 

 to the present day, allowing for errors of observation, which will ac- 

 count for all the slight differences exhibited in the course of the altern 

 ations of light. This star is in its minimum three quarters of a gradation 

 brighter than e ; in its maxinmm it resembles i of the same constellation 

 (Cepheus). It takes Id. 15h. to pass from the former to the latter ; but, 

 ou tile other ^■'i^nd, mo'-e than double that time, viz., 3d 18h., to change 



