There are many stars which exhibit periodic changes of brilliancy quite unaccounted for by any causes with which we are acquainted; these are termed variable stars. One of the most remarkable is o Ceti (Mira), which was first observed by David Fabricius in 1596. It appears about 12 times in 11 years; remains at its greatest brightness about a fortnight, when it sometimes equals in brilliancy a star of the 2nd magnitude; decreases during about 3 months till it becomes totally invisible; it remains so for about 5 months, and then gradually recovers its brilliancy during the remaining 3 months of its period. Its maximum brightness is not always the same, nor does it always increase or diminish by the same gradations. According to Hevelius, nothing was seen of it between October, 1672, and December, 1676. Algol (β Persei) is another well-known instance of this kind. For about 2d. 13h. it shines as an ordinary star of the 2nd magnitude. In about 3½ hours it is reduced to the 4th magnitude, and thus remains about 20 minutes; it then rapidly increases to the 2nd, and continues so for another period of 2d. 23h., when similar changes recur.