ASTRONOMY. 



[ADDENDA. 



surface of a litllo cone at the elliptic bane, T m n m' n' ; 

 ttill, however, the oone itself describing it* revolution 



Flf. . 



around the pole of the ecliptic, which is the " Precession 

 of the Equinoxes," as before stated. The motion in the 



Hiiiallrr cone, around its centre o, will evidently cause the 

 pole of the equator to be alternately on one aide and 

 another of its mean plane. The major axis of this ellipse, 

 m m', is directed in the plane which panes through its 

 axis, T o, and through the perpendicular, T K, to the 

 plane of the ecliptic : its amplitude is 19 '3". The smaller 

 axis of the ellipse is 14-4". The pole, P, makes a revolu- 

 tion of this ellipse in the space of eighteen years, return- 

 ing to m each time that the ascending node of the moon 

 is found at the spring equinox. To know, at any time 

 whatever, what is the position of the pole on the ellipse, 

 wo must imagine a circle described on the major axis, 

 m m', as a diameter, and to suppose that a point, z, 

 describes uniformly this circle in the direction of the 

 arrow, in a manner to come back again always to mat the 

 times at which the pole ought to be found there. At any 

 time whatever, the pole, P, is always situated at the 

 point of meeting the ellipse, m n m' n', with a perpen- 

 dicular to its major axis, drawn through the position that 

 the point z occupies at this instant. From this illustra- 

 tion the nature of nutation will be readily understood. 



ADDENDA TO CHAPTER III. 



OJCB of the most interesting speculations of Astrono- 

 mical Science, is that which relates to the physical con- 

 stitution of the sun ; and we shall, therefore, venture 

 to make some remarks in extension of those of Mr. 

 Breen, given at page 929, on this subject. 



The late advances made in Chemical and Optical 

 Science, have not only permitted us to have a more 

 accurate idea of the nature of solar light and heat, but, 

 to a large extent, we have also become acquainted with 

 some of the causes which produce them. The discovery 

 of the polarisation of light, enables us to poiut out some 

 of the characteristics of the solar luminous rays, as com- 

 pared with other sources of light ; but, since the year 

 1858, the observations of solar eclipses, and the discovery 

 of the spectrum analysis, have proved the existence of an 

 atmosphere encircling the sun, and shown that that 

 atmosphere must be in a state of intense ignition. We 

 will briefly point out how these facts have been ascer- 

 tained. 



At a subsequent page,* the reader will find an account 

 of some singular appearances, which have been noticed 

 during the eclipses of the sun that have occurred during 

 the last few years. And as our means of observing and 

 registering such appearances have improved, t the nature 

 of the red flames has been directly traced to the existence 

 of a solar atmosphere. At the time they have been 

 noticed, they could not have occurred on the surface of 

 the moon, because it is well known that our satellite 

 possesses no atmosphere ; and as there could be no other 

 cause assigned for them, they were accounted for in the 

 way we have named. 



Th discovery of the spectrum analysis, of which a full 

 account will be found at the end of the section on Light, 

 has satisfactorily set the question at rest. Our readers 

 will understand us better if we give a brief outline of the 

 manner of applying the spectrum method of investiga- 

 tion in this case. If a ray of white light be decomposed 

 by a glass prism, it is divided into seven colours, allord- 



SM p. M. 



t SM utlel* " CdMtlal Photography" Bwtloo Light. 



ing what is called the prismatic spectrum. Between 

 each extremity of the spectrum a vast number of dark 

 lines is distributed, which have definite positions for 

 every ray of light. If these be viewed by means of the 

 apparatus, illustrated and described at page 166, in the 

 section on Light, it will be found that the vapour of any 

 metal, passing through, or in combustion iu, any lumi- 

 nous flamo, will afford a distinctive coloured line for each 

 metal either of them being thus readily distinguished 

 by its own colour and position, in the spectrum so 

 alforded. If the reader will now peruse pages 107 and 

 168, in the section referred to, he will be prepared to see 

 the application of these wonderful discoveries to the in- 

 vestigation of the nature of the solar atmosphere, in 

 which, by these means, the existence of iron and other 

 metals, in a vaporous state, has been observed. Rea- 

 soning on these facts, as we have there stated, the Astro- 

 nomer Royal, Professor Airy, has pronounced a distinct 

 opinion that the surface of the sun is in a state of 

 incandescence ; and that immense bodies of vapour are 

 continuously arising from it, doubtless being the cause 

 of the intense light and heat emanating from that body. 

 This, perhaps, is one of the most interesting applications 

 of modern discovery with which we are acquainted. 

 SOLAR SPOTS. These curious appearances, which have 

 been generally considered to 

 exist ou the disc of the sun, 

 have generally attracted the 

 attention of observers of late 

 years. On July 25, 1862, a 

 remarkable spot was noticed, 

 having a length of 120,000 

 miles, and an area of 72,000,000 

 square miles. At its disap- 

 pearance, on August 4, a 

 distinct notch was atfonlr.l by 

 the photographs of it, which 

 seemed to prove that it was 

 due to an indentation of the solar disc. Fig. 40* illus- 

 trates the shape of the spot as drawn on July 27. ED. 



fig. w. 



