of AniareSf is'c. compared with other Stars. 453 



the twinkle, or bright apparent dilatation of the star. It seems 

 to occur only once in the course of five or six dilatacions, thougli 

 now and then oftener : I could not distinctly ascertain any re- 

 gular periods of it. The intervening colour of the light hardly 

 vai icd, except in the brilliancy of the twinklings. Being at Tun- 

 bridge Wells, on very high ground, I watched the phsenomenon 

 during great part of the night : sometimes the red colour was 

 very intense and bright, and was followed by an almost disap- 

 pearance of the star bv the a!)parent contraction. I do not 

 know whether I use terms sufficiently explanatory; but it is dif- 

 ficult, in describing phssnomena not much treated of before, to 

 select the most a!jpro|)riate terms. 1 hope I shall convey to 

 the reader a tolerably clear idea of the fact, and thatcoirespomliiig 

 observations will be made. I noticed this appearance six or 

 seven years ago, in the month of September; but it has certainly 

 increased since that time. 1 may observe also that the twinkling 

 of Anlares is greater than that of any other star visible at the 

 same time. As the above-mentioned alternation of colour is vi- 

 sible in several other stars, I have noticed them, with some of the 

 pccidiarities attending the changes of their light. 



1. Jntares in mean longitude 8' 7° H' 24"; mean south 



latitude 4° 30' 48". Has the greatest twinkling, and most 

 intense red colour alternating with white in the most irre- 

 gular manner. 



2. Betalgeus, or a. Ononis, mean longitude 2* 26° 14' 2G"; 



south latitude IG" 3' .")(/'. Alternations of red and yellowish, 

 not so distinctly marked as the last, and the times of the 

 red bear a greater proportion to those of the yellow. 



3. Aldebarati, or a Tauri, longitude 2^ 7° 16' 26"; south lati- 



tude 2 29' 16". Alternation still less conspicuous. 



As the above stars are in very different places in the heavens, 

 and as the stars about them do not show the least alternation; so 

 I think the change of colour caiutot depend on any thing in our 

 atmosphere, but must attend some alteration going on in the 

 alternating star. I must observe also, that this phenomenon is 

 seen only in the red stars ; it is faintly observable in Arclurwi, 

 but wholly imperceptible in Lyra, or the bri>iht star in the Eagle. 

 In Syrins it is faintly observable, but perhaps the least -so of all. 

 Syr'ius is reported by the old astronomers to have been a red 

 itar ; and perhaps the alternation I allude to, may be someh.ow 

 connected with the more permanent change of colour of varying 

 /stars. The phaenomenon is seen with refracting (and probably 

 also reflecting) telescopes, and certainly deserves the attention 

 of astronomers. I am, sir, &;c. 



.Sj.ii Lodnc Tunbridsc Wells, TliOMAS FoUSTliR. , 



June 20, 1817. 



Ff3 r. s. 



