Mr. Stewart on the Secular Change of Magnetic Dip at Kew. 235 



the sun. This material is not at once driven into the tail, bat usually 

 forms in front of the nucleus a dense luminous cloud, into which for 

 a time the bright matter of the nucleus continues to stream. In this 

 way a succession of envelopes may be formed, the material of which 

 afterwards is dissipated in a direction opposite to the sun, and forms 

 the tail. Between these envelopes dark spaces are usually seen. 



If the matter of the nucleus is capable of forming by condensation 

 a cloud-like mass, there must be an intermediate state in which the 

 matter ceases to be self-luminous, but yet retains its gaseous state, 

 and reflects but little light. Such a non-luminous and transparent 

 condition of the cometary matter may possibly be represented by 

 some at least of the dark spaces which, in some comets, separate the 

 cloud-like envelopes from the nucleus and from each other. 



Several of the nebulae which I have examined give a spectrum of 

 one line only, corresponding in refrangibility with the bright line of 

 the nucleus of the comet referred to in this paper. Other nebulas give 

 one and two fainter lines besides this bright line. Whether either or 

 both of these were also present in the spectrum of this comet I was 

 unable to determine. The light of the comet was feeble, and the pre- 

 sence of the continuous spectrum made the detection of these lines 

 more difficult. I suspected the existence of the brighter of these lines. 

 I employed different eyepieces, and also gave breadth to the bright 

 point by the use of the cylindrical lens, but I was not able to obtain 

 satisfactory evidence of more lines than the bright one already de- 

 scribed. 



In my paper " On the Spectra of the Nebulae," I showed that 

 this bright line corresponds in refrangibility with the brightest of 

 the lines of nitrogen. This line may perhaps be interpreted as an 

 indication that cometary matter consists chiefly of nitrogen, or of a 

 more elementary substance existing in nitrogen. 



The great varieties of structure which may exist among comets, as 

 well as the remarkable changes which the same comet undergoes 

 at different epochs, will cause all those who are interested in the ad- 

 vance of our knowledge of the cosmical relations of these bodies, and 

 of the gaseous nebulae, to wait with some impatience the visit of a 

 comet of sufficient splendour to permit a satisfactory prismatic ex- 

 amination of the physical state of cometary matter during the various 

 changes which are dependent upon the perihelion passage of the 

 comet. 



January 25. — Lieutenant-General Sabine, President, in the 

 Chair. 



The following communication was read : — 



" Note on the Secular Change of Magnetic Dip, as recorded at 

 the Kew Observatory." By Balfour Stewart, M.A., LL.D., F.R.S., 

 Superintendent of the Observatory. 



The President of this Society" has already called the attention 

 of the Fellows to the annual values of the magnetic inclination at 



