236 COSMOS 



be accurately calculate^ to within 10 to 15 minutes. It in 

 moreover remarkable that this star, after having increased 

 In light for about an hour, remains for nearly the same 

 period at the same brightness, and then begins once more per- 

 ceptibly to increase. Till very recently the duration of the 

 period was held to be perfectly uniform, and Wurm was able? 

 to present all observations pretty closely, by assuming it to 

 be 2d. 21 h. 48m. 58^s. However, a more accurate calculation, 

 in which was comprehended a space of time nearly twice 

 as long as that at Warm's command, has shown that the 

 period becomes gradually shorter. In the year 1784, it was 

 2d. 20h. 48m. 59'4s., and in the year 1842, only 2d. 20h. 

 48m. 55 '2s. Moreover, from the most recent observations 

 it becomes very probable that this diminution of the period 

 is at present proceeding more rapidly than before, so that 

 foi this star also a formula of sines, for the disturbance of 

 its period, will in time be obtained. Besides, this diminu- 

 tion will be accounted for, if we assume that Algol comes 

 nearer to us by about 2000 miles every year, or recedes from 

 us thus far less each succeeding year ; for in that case his 

 light would reach us as much sooner every year, as the de- 

 crease of the period requires; i. e. about the twelve thou- 

 sandth of a second. If this be the true cause, a formula of 

 sines must eventually be deduced. 



(3) x Cygni, R. A. 296 12', Decl. +32 32'. This star 

 also exhibits nearly the same irregularities as Mira. The 

 deviations of the observed maxima from those calculated for 

 a uniform period amount to 40 days, but are considerably 

 diminished by the introduction of a disturbance of 8-3- single 

 periods, and of another of 100 such periods. In its maximum 

 this star reaches the mean brightness of a faint 5th magni- 

 tude, or one gradation brighter than the star 17 Cygni. 

 The fluctuations, however, are in this case also very consi- 

 derable, and have been observed from 13 gradations below 

 the mean to 10 above it. At this lowest maximum the star 

 wuld be perfectly invisible to the naked eye, whereas, on the 

 contrary, in the year 1847, it could be seen without the aid 

 of a telescope for fully 97 days; its mean visibility extends 

 to 52 days, of which, on the mean, it is 20 days on the 

 increase, and 32 on the decrease. 



(4) 30 Hydra Hevelii, R. A. 200 23', Decl. 22 30'. Of 

 this star which, from its position in the heavens, ie only 



