Galilei, impatient to obtain ocular evidence of what he called the "structure of the universe," soon provided himself with a second instrument, and on directing it towards -the moon, this luminary became immediately stripped of the character of geometrical per fection, absurdly attributed to all the celestial bodies by the school men, according to whom they were all perfectly round, self-luminous, and uncorrupted by any terrestrial tarnish.
The more obscure parts of the lunar surface, which they imagined had arisen from some earthly taint consequent ou the proximity of the moon, being now rendered distinctly visible, taught Galilei that the surface of the moon was irregular and uneven, having mountains and valleys of much greater extent, in proportiou, than those on our globe; the faint light on the darkened portion of the moon's surface he recognised to be the reflection of the sun's rays from the earth ; the luminous isolated points near her inner border, and the jagged outline of that border, showed the mat inequalities on her surface, since the mountain-tops would be illuminated by the aim, while the sides and base would lie in obscurity, in consequence of the con vexity of tbe surface. In pursuing these observations, ho found that the moon terra towards the earth the same face constantly, so that nearly a hemisphere of her surface can never be visible to us. From this remarkable fact he does not appear to have drawn the inevitable consequence, that the time of her rotation round her own axis, and the time of a revolution round the earth, must be exactly equal.
Lagrange afterwards suggested that this effect was primitively caused by the determination of the lunar figure, in which the heavier part being originally accumulated towards the attracting primary, the moon, in its revolution, would always have a tendency to fall towards the heavier side so determined. Galite! subsequently observed the librationa of the moon, by which small portions of her more distant hemisphere are alternately brought in view ; but he was not in a situation to give a satisfactory explanation of the cause, from the imperfection of theoretical astronomy. The idea which was suggested from the appearance of ocean. and continents, mountains and valleys, on the moon, that she might be habitable, overwhelmed the school men with horror, and struck the religious with alarm.
On examining the nebula, and particularly the Milky Way, with his plass, he perceived that they were composed of myriads of stare, or, in the language of Milton, "powdered with stars." It may be remarked in pealing, that Milton visited Galilei, and entertained the highest opinion of his philosophy, to which he makes several beautiful allusions in his Paradise Lost.'
The planet Jupiter furoished matter for still greater wonder. (Utile! perceived three very small stars eastward of the planet, and close to its disc ; two of them, on a subsequent observation, had distinctly changed position to the westward : he soon perceived that they were astellites ; and shortly afterwards be discovered the fourth.
The strength which this discovery gave to the Copernican system, from the analogy with our moon, however gratifying to Galilci in a speculative point of view, did not prevent his ever-active mind from perceiving its great practical -importance in the question of deter mining longitudes at sea; but it was reserved for a future age to bring this and other methods to a degree of perfection then impracti cable. The theory of astronomy and the construction of chrono meters were, at that time, in a most imperfect state ; and though Galilei offered his services to Spain, then a great maritime power, it is doubtful whether be would not have had cause for regret if the wished-for arraugemeut had taken place. The manner in which he was wailed after this discovery must have caused him amusement rather than chagrin: some would not look through his glass to be convinced ; one Horky asserted that be bad used the telescope, and that he saw nothing of the kind ; one thought it odd that nature should give satellites to Jupiter for no purpose but to immortalise the Medici family (for Galilei had denominated them Medicean stars, in honour of his patron). Some time after, his opponents found out five satellites for Jupiter instead of four; while one had the impudence to say that he actually asw nine satellites. (1610.) On examining Saturn with the telescope be perceived his ring, or rather rings (as Sir IV. Herschel has since shown), but viewing it in perspective, be took the lateral portions for two small stars, which induced him to announce in transposed letters the following sentence — "Altlastmum Ptanetam teractelnum observed." (The most distant planet I have observed to be threefold.) Ifuygliens was the first who corrected this error; though it is remarkable that the occasional disappearance of the supposed lateral planets, which arose from the relative change of the position of the ring, which so much astonished Galilei, had not suggested to him the correct nature of the phenomenon : we must however remember the great imperfections of the first constructed telescopes.