Notices respecting New Books. 149 



the following observations, which indicate a probability that the sun 

 itself belongs to the class of variable stars. 



" Ignorant as we are, however, both of the cause of solar and 

 stellar light, and of the conditions which may influence its amount 

 at different times, the law of regular periodicity is one which ought 

 not to be too hastily generalised, and at all events there is enough 

 of slow and gradual change of lustre in many stars, since the earlier 

 ages of astronomy, to refute all a priori assumption as to the possible 

 length of the cycle of variation of any particular star. The subject 

 is one of the utmost physical interest. The grand phenomena of 

 geology afford, as it appears tome, the highest presumptive evidence 

 of changes in the general climate of the globe. I cannot otherwise 

 understand alternations of heat and cold so extensive as at one epoch 

 to have clothed high northern latitudes with a more than tropical 

 luxuriance of vegetation ; at another to have buried vast tracts of 

 middle Europe, now enjoying a genial climate, and smiling with fer- 

 tility, under a glacier crust of enormous thickness. Such changes 

 seem to point to some cause more powerful than the mere local dis- 

 tribution of land and water (according to Mr. Lyell's views) can well 

 be supposed to have been. In the slow secular variations of our 

 supply of light and heat from the sun, which in the immensity of 

 time past may have gone to any extent, and succeeded each other in 

 any order, without violating the analogy of sidereal phenomena 

 which we know to have taken place, we have a cause, not indeed 

 established as a fact, but readily admissible as something beyond a 

 bare possibility, fully adequate to the utmost requirements of geology *. 

 A change of half a magnitude in the lustre of the sun, regarded 

 as a fixed star, spread over successive geological epochs, — now pro- 

 gressive, now receding, now stationary, according to the evidence of 

 warmer or colder general temperature which geological research has 

 disclosed, or may hereafter reveal, — is what no astronomer would 

 now hesitate to admit as in itself a perfectly reasonable and not im- 

 probable supposition. Such a supposition has assuredly far less of 

 extravagance about it than the idea that the sun, by its own proper 

 motion, may, in indefinite ages past, have traversed regions so 

 crowded with stars as to affect the climate of our planet by the in- 

 fluence of their radiation. Nor can it be objected that the character 

 of a vera causa is wanting in such an hypothesis. Of the exciting 

 cause of the radiant emanations from the sun and stars we know 

 nothing. It may consist, for aught we can tell, in vast currents of 

 electricity traversing space (according to cosmical laws), and which, 

 meeting in the higher regions of their atmospheres with matter pro- 

 perly attenuated, and otherwise disposed to electric phosphorescence, 

 may render such matter radiant, after the manner of our own aurora 

 borealis, under the influence of terrestrial electric streams. Or it 



* " Many phenomena in natural history seem to point out some past 

 changes in our climates. Perhaps the easiest way of accounting for them 

 may be to surmise that our sun has formerly been sometimes more and 

 sometimes less bright than it is at present." — W. Herschel on the Changes 

 that happen to the Fixed Stars. Phil. Trans., 1796, p. 186. 



