densities, both acted upon by the same exterior planetary electrical currents. It is certainly true of double stars, and probably so of all the others. Of course such enormously massive double suns presuppose enormous planets, rotating around them at enormous distances; but when we compare the distance of our own satellite, the moon, from the earth with the distance of Neptune from the sun, and consider that the light of the sun will reach Neptune in about four hours, and then compare this distance with the inconceivable distances of space requisite to retard and merge all radiant energy into the diffused molecular energy of position, our wonder will cease. We have also to consider those single stars which (see Appleton's Cyclopædia, article "Star") are variable in their brilliancy. "These stars may be divided into periodic variables, irregular variables, and temporary stars. Periodic variable stars are those which undergo increase and diminution of light at regular intervals. Thus, the star Mira, or Omicron of Cetus, varies in lustre, in a period of three hundred and thirty-one and one-third days, from the second magnitude to a faintness such that the star can only be seen with a powerful telescope, and thence to the second magnitude again. It shines for about a fortnight as a star of the second magnitude, and then remains invisible for five months, the decrease of lustre occupying about three months, the increase about seven weeks. Such is the general course of its phases. It does not always, however, return to the same degree of brightness, nor increase and diminish by the same gradations; neither are the successive intervals of its maxima equal. From recent observations and inquiries into its history, the mean period would appear to be subject to a cyclical fluctuation embracing eighty-eight such periods, and having the effect of gradually lengthening and shortening alternately those intervals to the extent of twenty-five days one way and the other. The irregularities in the degree of brightness attained at the maximum are probably also periodical.... It suggests a probable explanation of these changes of brightness, that when the star is near its minimum, its color changes from white to a full red, which, from what we know of the spectra of colored stars, seems to indicate that the loss of brightness is due to the formation of many spots over the surface of this distant sun.