YOL. LXXXVI.] PHILOSOPHICAL TRANSACTIONS. 723 



which we are all immediately concerned. Who, for instance, would not wish to 

 know what degree of permanency we ought to ascribe to the lustre of our sun ? 

 Not only the stability of our climates, but the very existence of the whole animal 

 and vegetable creation itself, is involved in the question. Where can we hope 

 to receive information on this subject but from astronomical observations? If it 

 be allowed to admit the similarity of stars v/ith our sun as a point established, 

 how necessary will it be to take notice of the fate of our neighbouring suns, in 

 order to guess at that of our own ! That star which among the multitude we 

 have dignified by the name of sun, to-morrow may slowly begin to undergo a 

 gradual decay of brightness, like (3 Leonis, « Ceti, y. Draconis, ^ Ursae majoris, and 

 many other diminishing stars that will be mentioned in my catalogues. It may 

 suddenly increase, like the wonderful star in the back of Cassiopea's chair, and 

 the no less remarkable one in the foot of Serpentarius ; or gradually come on 

 like |3 Geminorum, (3 Ceti, i^ Sagittarii, and many other increasing stars, for 

 which I also refer to my catalogues. And lastly, it may turn into a periodical 

 one of 25 days duration, as Algol is one of 3 days, ^ Cephei of 5, |3 Lyras of 6, 

 »i Antinoi of 7 days, and as many others are of various periods. 



Now, if by a proper attention to this subject, and by frequently comparing 

 the real state of the heavens with such catalogues of brightness as mine, it should 

 be found that all, or many of the stars which we now have reason to suspect to 

 be changeable, are indeed subject to an alteration in their lustre, it will much 

 lessen the confidence we have hitheito placed on the permanency of the equal 

 emission of light of our sun. 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 been formerly sometimes more 

 and sometimes less bright than it is at present. At all events, it will be 

 highly presumptuous to lay any great stress on the stability of the present 

 order of things ; and many hitherto unaccountable varieties that happen in 

 our seasons, such as a general severity or mildness of uncommon winters or burn- 

 ing summers, may possibly meet with an easy solution in the real inequality of 

 the sun's rays. 



A method of ascertaining the quantity or intenseness of solar light might be 

 contrived by some photometer or instrument properly constructed, which ought 

 probably to be placed on some high and insulated mountain, where the influence 

 of various causes that afiect heat and cold, though not entirely removed, would 

 be considerably lessened. Perhaps the thermometer alone might be sufficient. 

 For though the lu&tre of the sun should be the chief object of this research, yet, 

 as the effect of light in producing expansion in mercury seems to be intimately 

 connected with the quantity of the incident solar rays, it may be admitted that 



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