642 POPULAR SCIENCE MONTHLY. 



laid down to govern them. Thus, during the period from 1782 to 1800 

 it was from thirteen to twenty-four days late. In 1812 it was thirty- 

 nine days late. From 1845 to 1856 it was on the average about a month 

 too early. Several recent maxima, notably those from 1895 to 1898, 

 again occurred late. Formulae have been constructed to show these 

 changes, but there is no certainty that they express the actual law of 

 the case. Indeed, the probability seems to be that there is no invariable 

 law that we can discover to govern it. 



Argelander fixed the length of the period at 331.9 days. More 

 recently, Chandler fixed it at 331.6 days. It would seem, therefore, to 

 have been somewhat shorter in recent times. It was at its maximum 

 toward the end of October, 1898. We may, therefore, expect that fu- 

 ture maxima will occur in July, 1901; June, 1902; May, 1903; April, 

 1904, and so on, about a month earlier each year. During the few 

 years following 1903 the maxima will probably not be visible, owing to 

 the star being near conjunction with the sun at the times of their 

 occurrence. The most plausible view seems to be that changes of a 

 periodic character, involving the eruption of heated matter from the 

 interior of the body to its surface, followed by the cooling of this mat- 

 ter by radiation, are going on in the star. 



The star Algol, or Beta Persei, as it is commonly called in astronomi- 

 cal language, may, in northern latitudes, be seen on almost any night of 

 the year. In the early summer we should probably see it only after 

 midnight, in the northeast. In late winter it would be seen in the 

 northwest. From August until January one can find it at some time 

 in the evening by becoming acquainted with the constellations. It 

 is nearly of the second magnitude. One might look at it a score of 

 times without seeing that it varied in brilliancy. But at certain 

 stated intervals, somewhat less than three days, it fades away to nearly 

 the fourth magnitude for a few hours, and then slowly recovers its 

 light. This fact was first discovered by Goodrick in 1783, since which 

 time the variations have been carefully followed. The law of variation 

 thus defined is expressed by a curve of the following form: 



u u 



Fig. 2. Law or Variation of a Stab of the Algol Type. 



The idea that what we see in the star is a partial eclipse caused by 

 a dark body revolving round it, was naturally suggested even to the 

 earliest observers. But it was impossible to test this theory until re- 

 cent times. Careful observation showed changes in the period be- 

 tween the eclipses, which, although not conclusive against the theory, 

 might have seemed to make it somewhat unlikely. The application 

 of the spectroscope to the determination of radial motions, enabled 



