On favourable occasions the brilliancy of Jupiter is very considerable; so much so that it rivals Venus and Mars. And besides this, there appears to be something special in the nature of Jupiter’s surface, for not only does it seem to radiate a much larger proportion of the solar light which falls on it than do the planets generally, but some observers have expressed the opinion that it possesses inherent light of its own. Speculations, however, such as this must always be received with reserve, because of the evident difficulty of making sure of the facts on which they must be based. One thing, however, seems less open to doubt. Bearing in mind the small amount of heat which reaches Jupiter from the Sun, there is reason to infer that the clouds which certainly exist on Jupiter must owe their origin to the influence of some other heat than solar heat; in other words that Jupiter possesses sources of heat within itself. Jupiter has satellites, 5 in number. The discovery of four of these, was one of the first fruits of the invention of the telescope, for they were found by Galileo in January, 1610. The 5th satellite is so small that it escaped notice until as recently as 1892, having been discovered on September 9 of that year by Professor Barnard, with the great Lick telescope in California. It is, however, so minute that one can count on one’s fingers the telescopes capable of showing it. The four old satellites of Jupiter shine as stars of about the 7th magnitude; in other words, they are sufficiently bright to be visible with telescopes however small: indeed several instances are on record of persons gifted with very good sight, having been able to see them with the naked eye. For the study of their physical appearance very powerful optical assistance is necessary, but their movements are so rapid, and the phenomena which result from those movements are so interesting, that these bodies may be considered to occupy the first place in the stock-in-trade of every amateur astronomer, who lays himself out for planet-gazing, with the object of profiting himself or his friends. The phenomena here alluded to are known as eclipses, transits, and occultations. The four old satellites do not bear any names, but are numbered from the innermost outwards, and are always alluded to by their numbers as I, II, III and IV.