25 



One consequence of this state of things is, that they can never 

 be all three eclipsed at the same time. Laplace discovered these 

 laws, and demonstrated that they are necessary results from 

 the mutual actions of the satellites, and that the same cause 

 tends to perpetuate their operation. 



Delambre next made these theorems of Laplace the basis of 

 his researches, and occupied several years in the composition of 

 ecliptic tables founded upon them. — Arising out of these re- 

 searches, it was found by most careful observations, that when 

 the earth is in that part of her orbit which is nearest to Jupi- 

 ter, the eclipses appear to happen 8m. ]3s. sooner than the 

 tables predicted ; and when in that part of her orbit which is 

 the farthest from him, 8m. 13s. later than the tables predicted ; 

 from the mean of 6,000 eclipses observed by Delambre, this 

 disagreement between observation and calculation was satis- 

 factorily settled at 8m. 13s. while both were considered equally 

 correct. 



We learn therefore from the foregoing results, that by a law 

 of nature the three first satellites of Jupiter can never become 

 eclipsed all at the same time. The simultaneous disappearance, 

 however, does not apply to eclipses properly so called, that is 

 to say, the passage of the satellite through the cone of the 

 planet's shadow, but it may happen that all are either eclipsed, 

 occulted, or projected on the body of the planet, in which 

 cases they are equally invisible, except under peculiar circum- 

 stances with good telescopes. 



Instances from observation of Jupiter being denuded of 

 his satellites are recorded as follows : — The first by Mr. 

 Molyneux, in his Optics, November 2nd (old style) 1681 ; by 

 Sir William Herschel, May, 23rd, 1802 ; by Mr. Wallis, 

 April 15th, 1826 ; by Mr. Griesback and Mr. Daws, Septem- 

 ber 27th, 1843. 



The first three satellites (lo, Europa, and Ganymede) move 

 nearly in the plane of Jupiter's equator, in orbits very nearly 

 circular ; on this account they pass through the planet's 

 shadow, and are eclipsed every revolution, but the inclination 

 of the fourth satellite (Callisto) is about 8°, and its eccentricity 

 large, so that when this satellite is seen to pass across 

 Jupiter's body, or behind it, the apparent path is fre- 

 quently very far from the centre. The Eev. Eobert Maiu 

 records one such transit when the satellite merely grazed the 

 upper part of the disc. 



The announcement of the Astronomer Eoyal, to the Astro- 

 nomical Society, of the approaching and rare phenomena of 

 the concealment of Jupiter's four satellites " on August 21st," 

 will occur to Hobart Town mean time as follows: —On the 



