GALILEO. in 



at sea. The reader, even if unacquainted /with astronom^ will have 

 no difficulty in understanding the principle upofi \thjch Galileo thought 

 of utilizing the newly-discovered planets. Supposing that the exact'' 

 instant of time at which the Jovian satellites severally plunge into or 

 emerge from the shadow of their primary had been, emulated twelve 

 months beforehand, and that the days, minutes, and seconds at which 

 these occurrences would be witnessed at Florence had been registered, 

 a mariner at sea who wished to ascertain his longitude would observe 

 some particular eclipse which had been predicted for that night. He 

 would note the hour, minute, and second at which the eclipse took 

 place ; for we may suppose him to have a watch which he had set to 

 the true time by observations of the sun the same day. The difference 

 between the time indicated by his watch and the time at which, ac- 

 cording to the clocks at Florence, the eclipse takes place, would give 

 him the longitude. --The earth turns through 360 in twenty-four hours, 

 that is, through 15 in each hour; so that the time of noon is later by 

 one hour at places 1 5 to the west of a given place, and so on in like 

 proportion. Galileo's plan was unexceptionable in principle, but there 

 are practical difficulties in observing the satellites of Jupiter from the 

 deck of a ship at sea ; and, further, the mariners of that time were 

 not provided with timepieces or watches upon which reliance could 

 be placed. But for these merely mechanical obstacles, the world 

 would have witnessed, at a period when men had not entirely ceased 

 to draw horoscopes, the realization, in a nobler sense, of the astro- 

 loger's vain dream of planetary influences ; for the satellites of Jupiter, 

 invisible to the unassisted vision, might be said by their aid to naviga- 

 tion to play no inconsiderable part in directing the destinies of our 

 race. 



The next planet which received Galileo's attention was Saturn, but 

 his telescope was not sufficiently powerful to show him distinctly the 

 real configuration of this wonderful planet. He interpreted the ap- 

 pearances presented in his glass as indicating that Saturn was a triple 

 planet. " I have," he says, " observed with wonder that Saturn is not 

 a single star, but three together, which, as it were, touch each other ; 

 they have no relative motion, and are constituted in this form oQo> 

 the middle being somewhat larger than the lateral ones. If we ex- 

 amine this with a glass which magnifies the surface less than a thou- 

 sand times, the three stars do not appear. very distinctly, but Saturn 

 then presents an oblong appearance, like the form of an olive, thus 

 CD- I have now discovered a court for Jupiter, and for this old man 

 two servants who aid his steps and never quit his side." Galileo's 

 next discoveries were {he solar spots, or rather, he for the first time per- 

 ceived them in their true relations, for these spots are occasionally so 

 large that they become visible without the aid of a telescope, and ob- 

 servations of their appearance in this way are upon record. As Galileo 

 noticed that the spots moved across the sun's disc, he was at first in- 



