5l6 PHILOSOPHICAL TRANSACTIONS. [ANNO ]784. 



than 1 cubic inch. If we suppose the filings to be from steel, 1000 grs. of steel 

 containing 8 of plumbago, we may have about 2.5 of fixed air, or about 1.5 cubic 

 inch, and this is the strongest supposition, and the most favourable to Mr. 

 Cavendish. What shall we then say, if we consider that these filings were mixed 

 with copper or brass, which contain no plumbago ? and, above all, that plum- 

 bago cannot be supposed decomposable by red precipitate, since even the nitrous 

 acid cannot decompose it ? 



5thly. With regard to the power which nitrous selenite has of absorbing fixed 

 air, I must allow the experiments of Mr. Cavendish to be just and agreeable to 

 my own; but it only follows, that when fixed air is in its nascent state, it is more 

 absorbable. Thus many metallic calces take it from alkalis in its nascent state, 

 though in other circumstances they will take none. Lastly, the permanence of 

 a mixture of nitrous and common air, made over mercury, cannot be attributed 

 to nitrous vapour, as vapour is not elastic in cold ; besides, I have often made the 

 mixture without producing any such durable vapour, and this will always happen, 

 when the nitrous, air is made from nitrous acid sufficiently diluted. 



XVI I. On a Method of Describing the Relative Positions and Magnitudes of the 

 Fixed Stars; with some Astronomical Observations. By the Rev. F. IVollastony 

 LL.B.F.R.S. p. 181. 



From some alterations which have of late years been discovered, in the relative 

 positions and apparent magnitudes of a few of the stars we call fixed, it seems not 

 unreasonable to conclude, that there may be many changes among others that 

 we little suspect. This thought has led me into a wish, that some method were 

 adopted to detect such motions. The first idea which occurred to me was, to 

 make a proposal to astronomers in general , that each should undertake a strict 

 examination of a certain district in the heavens ; and, not only by a re-examina- 

 tion of the catalogues hitherto published, but by taking the right ascension and 

 declination of every star in their several allotment, to frame an exact map of it, 

 with a corresponding catalogue ; and to communicate their observations to one 

 common centre. This is what I could be glad to see begun. Every astronomer 

 must wish it, and therefore every one should be ready to take his share in it. 

 Such a plan, undertaken with spirit, and carried on gradually with care, would, 

 by the joint labours and emulation of so many astronomers as are now in Europe, 

 produce a celestial Atlas far beyond any thing that has ever yet appeared. 



But this would be a work of time, and not within the compass of every one. 

 What I mean now to propose is more immediate ; and not out of the reach of any 

 who amuse themselves with viewing the heavenly bodies. Meridian altitudes and 

 transits can be taken but once in 24 hours ; and, though accurate, are therefore 

 tedious. Neither can any re-examination of them be made, but with the same 



