BIBLIOGRAPHY OF HERSCHEL's WRITINGS. 607 



Herschel, "W.: Synopsis of the Writings of— Contiuncd. 



A. D. Vol. P. 



1815 105 345 Consideration of fhc 2n-inciples hy whirh the periodical revolution of the 

 satellites may be obtained from the olmrred an<ilef> of position. 

 348 The periodical rcvoli(tionfi of the satellites detirmined. 



350 The first satellite I'Titania'] niakos a syiiodical revolution in B'l IG^ SC" 



5.2= and the second lOberon] in 13'' 11'' 8"' 5'J«. 



351 Explanation of the identifying method. 



355 I. With the light of my 20-foot telescope the first satellite generally 



becomes invisible at a distance of [18"] from the planet; and 



II. The second at abont [24"]. 



III. An interior satellite cannot be seen two nights in snccession. 



356 IV. Exterior satellites that are very faint when at their greatest 



elongation can hardly ever be seen at any other time when the 

 orbits are contracted. 



V. lTitania'\ is probably larger than \_Oberon]. 



VI. Both are subject to great variations of light [not owing to the 

 changeable clearness of the air]. 



VII. This may be due to a rotation on their axes, or from atmos- 

 pheres. 



VIII. I have supposed the distances of the first and second satellites 

 to be 36" and 48" respectively. 



IX. The existence of additional satellites has been considered already 

 in Phil. Trans., 1798, p. 59. 



358 An interior satellite. 



359 Addition [in regard to an interior satellite]. 



360 An intermediate satellite. An exterior satellite; and Addition. 



361 More distant satellites ; ami Addition. 



362 Plate XVI, diagram. 



1817 107 302 Astronomical Observations and Experiments tendinfi to investigate the local 

 Arrangement of the Celestial Bodies in Space and to determine the Extent 

 and Condition of the Milky Way. By Sir William Herschel, Knt. 

 Gnelp., LL. D., F. R. S. Read Jane 19, 1817. 



302 The construction of the heavens can only be known when we have 



the situation of each body defined by its three dimensions. Of 

 these three the ordinary catalogues give but two, l.-aving the dis- 

 tance or profundity undetermined. 



303 The method of parallaxes has given the distance of the sun, planets^ 



etc The parallax of the stars has also received attention. A\ ith 

 regard to more distant objects, as small stars, compressed clusters, 

 ami nebuhe, these methods can give us no assistance. 



303 I. Of the local situation of the stars of the Heavens. 



304 It is evident that we cannot mean to affirm that the stars of the fifth 



sixth, and seventh magnitudes are really smaller than those of he 

 first, second, or third, and that wo must ascribe the cause of the 

 dificTence in the apparent magnitudes of the stars to a difference in 

 their relative distances from us. On account of the great number 

 of stars in each class we must also allow that the stars of each suc- 

 ceedin.r magnitude beginning with the first, are, one with another, 

 further from us than those of the magnitude immediately preceding 

 The relative magnitudes give only relative distances and can aftord 

 no information as to the real distances at which the stars are placed. 

 304 II. Of a standard by tvhieh the relative arrangement of the stars may 



Airnirt of reference for the arrangement of the stars may be had 



