166 SPECIAL RESULTS IN THE URANOLOGICAL PORTION 



shews a less degree of light for between 7 and 8 hours, 

 in the course of which it descends to the 4th magnitude. 

 The decrease and increase of brightness are not quite 

 regular, but, proceeding more rapidly near the minimum, 

 enable the moment of least brightness to be determined 

 to within 10 or 15 minutes. It is remarkable that, after 

 increasing in light for the space of an hour, it remains for 

 about the same time at almost exactly the same degree of 

 brightness, after which it again begins to increase sensibly. 

 Hitherto the length of the period has been supposed to 

 be perfectly uniform, and Wunn was able to represent all 

 the observations well, by taking it at 2d. 21h. 48m. 58*5s. 

 A more exact calculation, with an interval almost twice as 

 great as that which Wurm had at his command, has, 

 however, shewn that the period is becoming gradually 

 shorter. In 1784 it was 2d. 20h. 48m. 59'4s., and in 

 1842 only 2d. 20h. 48m. 55'2s. The latest observations 

 render it very probable, also, that this decrease of the 

 period is taking place more rapidly than before, so that, 

 for this star also, there will in time be derived a formula 

 of sines for the perturbation of the period. The present 

 shortening of the period might be explained by the as- 

 sumption, that Algol either approaches us nearer every 

 year by about 2000 geographical miles, or recedes from us 

 that quantity less each year than the preceding ; as in 

 such case the light would reach us each year as much 

 sooner as the diminution of the period requires, i. e. 

 about 12 thousandth parts of a second. Should this be 

 the true reason, there would naturally be deduced in time, 

 a formula of sines. 



3. x Cygni, E. A. 269 12'] DecL + 32 to 32'. This 



