OP ARTS AND SCIENCES. 35 



page 6 for the equivalent diameter of the system, using as a unit 

 the radius of the star. We thus find, b = 0.00933 aP-i = 0.00933 

 X 4.60 (0.00785)-* = 1.087. Accordingly, a body having the density 

 of the Sun, and a diameter but little more than half that of Algol, 

 would give the observed time of revolution to the satellite. If, there- 

 fore, the velocity is remarkable, it is remarkable that it is not greater. 

 If the satellite of Algol has a diameter of 0.7 G4, and its density equals 

 that of the primary, its relative mass will be 0.446. The two bodies 

 combined would form a sphere having a radius of 1.130 and a diameter 

 of 2.260. This is 2.08 times that of the equivalent diameter, and shows 

 that the average density can be only 0.11 of that of the Sun, or about 

 one seventh of that of water. 



It may be noted that the density affords a means of distinguishing 

 between a satellite and a spherical cloud of meteors. If the individual 

 meteors were very minute, they might completely cut off the light, 

 and yet bear a very small ratio in volume to the space between them. 

 Accordingly, if the density of the eclipsing body could be shown to be 

 very small, we might infer that it was composed of meteorites. In 

 this case the motion of Algol would be insensible, as seen in the 

 spectroscope. 



The observed times of minima of Algol seem to show that its period 

 has undergone a diminution during the last century. Such a change 

 is easily explained on the theory of a secondary satellite. The disturb- 

 ance caused by a third body, or by a resisting medium, might very 

 sensibly vary the period from year to year. The law of this change 

 is not yet known, but its nature is shown in Table XII. The minima 

 are distinguished by successive numbei's, JS, that occurring on Jan. 1, 

 1800, being designated as 0. Those preceding 9000 have been ar- 

 ranged in groups of 500 each. Since 1870 the observations of each 

 year are grouped together. The successive columns of the table give 

 a current number, the mean of the numbers of the minima, the corre- 

 sponding year and tenth and the number of minima included in the 

 group. In the last nine groups, which relate to a single year, the 

 minimum corresponding to opposition is used, instead of the mean of 

 those observed. 



The first eleven sets were observed by various astronomers ; sets 

 12 to 18 were made by Argelander ; sets 19 to 21 by Schonfeld ; and 

 sets 22 to 30, by Schonfeld and Schmidt. Sets 18 and 19 relate to 

 the same period of 500 revolutions from 7500 to 8000. The fifth 

 column is found by subtracting from the observed time that given by 

 the formula of Schonfeld on page 94 of his memoir, — 



