W. B. Scott — Some New Forms of the Dinocerata. 303 



he estimated the brightness of the new star on November 12 

 to be the same as that of the star of comparison. Such a rela- 

 tive estimate is good, but the absolute magnitudes are uncertain. 

 The limit of visibility in a 26-inch telescope is 16'3 magnitudes, 

 and the nova is now so near this limit that accurate measures 

 can no longer be made. 



The great nebula of Andromeda is easily visible to the naked 

 eye, and doubtless it was known to the astronomers of very 

 ancient times. Those astronomers watched the heavens with 

 unaided vision much more carefully than do modern astrono- 

 mers, and they were far better acquainted with the constella- 

 tions. The old astronomers had a theory that this nebula was 

 variable both in form and brightness. They had poor means 

 of judging of its form, but it is possible that their estimates of 

 brightness may be more trustworthy, and that our new star may 

 be an old variable which has appeared before, causing the 

 nebula apparently to vary in brightness. One of the best de- 

 scriptions of this nebula is by an astronomer of the middle 

 ages, Simon Marius in 1612, who says it resembles a candle 

 shining through a horn. It has a bright nucleus, around which 

 is the soft, fleecy-looking nebulous matter, shading off very 

 gradually from the center. This nebula has an angular extent 

 of two degrees. No parallax has yet been found for a nebula ; 

 but if we suppose them to be as distant as the brightest of the 

 fixed stars, and to have a parallax of a quarter of a second of 

 arc, we may get some idea of the enormous extent of such a 

 nebula as the great one in Andromeda. Making the preceding 

 assumptions, its diameter will be about a thousand times as 

 great as the distance of Neptune from the sun. Probably this 

 estimate is less than the real diameter. That changes go on in 

 such vast bodies we cannot doubt, for we see motion and 

 change everywhere, but the distances are so great that small 

 changes would pass without notice by astronomers on our 

 earth. For this reason and also from the difficulty of making 

 accurate drawings of such indefinite objects, we have as yet 

 hardly any proof of changes in the forms of nebulas. 



U. S. Naval Observatory, 1886, Feb'y 12tb. 



Aet. XXIX. — On Some New Forms of the Dinocerata; by 



W. B. Scott. 



In 1875, Professor Cope established the Amblypoda as an 

 order of hoofed mammals, including as its two sub-orders the 

 Coryphodons of the Wahsatch Eocene and the Dinocerata of 

 the Bridger. This association of animals so divergent in ap- 

 pearance rests' more especially upon the structure of the feet 



