314 Royal Astronomical Society : Sir J. F. W. Herschel 



Jan. 7, 1840. " Procyon, Orion, Aldebaran, form a succession 

 by nearly equal steps." " Upon the whole, I think it may be 

 stated, that in the interval from November 26 to the present date 

 (January 8), Orion has sustained a loss of nearly half its light. It 

 may easily be supposed that a diminution, thus evidently still in 

 rapid progress, will, in no long time, carry down the rank of this 

 star below that of Aldebaran, and that the confirmation or disap- 

 pointment of this expectation is awaited with no small interest." 



The author concludes with the following remarks : 



" The subject of variable and periodical stars has been of late 

 rather unaccountably suffered to lie dormant ; a state of neglect in 

 which, as I have already observed, it ought not to be suffered to 

 remain, and from which I have endeavoured to rescue it on two 

 former recent occasions, by pointing out the stars a Hydrce and 

 a Cassiopeia, both large and conspicuous stars, as belonging to the 

 latter class. A periodical change, however, existing to so great an 

 extent in so large and brilliant a star as a Orionis, cannot fail to 

 awaken attention to the subject, and to revive the consideration 

 of those speculations respecting the possibility of a change in the 

 lustre of our sun itself which were put forth by my father. If there 

 really be a community of nature between the sun and fixed stars, 

 every proof that we obtain of the extensive prevalence of such 

 periodical changes in those remote bodies, adds to the probability 

 of finding something of the kind nearer home. It is only in com- 

 paratively very recent meteorological observations that we can ex- 

 pect to find that precision in the determination of temperatures 

 which is necessary to establish the absence or presence of periodical 

 change in the intensity of solar radiation ; and if the period be not 

 annual (as there is no reason why it should be), the usual mode of 

 combining observations of temperature followed by meteorologists 

 is altogether inappropriate to the research, which can only be carried 

 on either analytically, by the introduction of a periodical term with 

 unknown coefficient, epoch, and period, or graphically, by projecting 

 in a continuous curve the mean daily temperatures during a long 

 series of years. For the detection of a period of great length, ex- 

 tending over more than a year, the continued observation of the 

 temperature of the water a few feet below the surface in open sea, 

 under the equator, on the principles pointed out by M. Arago in 

 his instructions for the voyage of the Bonite, would suffice. But 

 we are far from possessing as yet sufficient records of such obser- 

 vations to be worth discussing in this point of view. Such obser- 

 * vations must of their nature be casual. Even granting that in every 

 ship which traversed the equator the requisite observations were 

 made, the identity of their thermometric standards would be still 

 open to question. 



" The assiduous observation in fixed physical observatories of 

 the temperature of the earth, at several depths below the surface, 

 extending from three to thirty feet an element which we know to 

 be (in its mean amount) solely dependent on solar radiation 

 would be in every respect more immediately and practically appli- 

 cable to the inquiry, and we may expect to see it carried out into 



