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HISTORY OF PHYSICAL ASTRONOMY.

sufficiently powerful to exhibit them. According to Riccioli, they werefirst observed by Zucchi, an Italian Jesuit *. On the 17th of May, 1630,that individual perceived two belts on the disk of the planet. In 1633,Fontana perceived three belts. In 1648, Grimaldi was unable to perceivemore than two. He was the first person who remarked that the directionof the belts is parallel to the plane of Jupiter ’s orbit. In addition to thebelts which surround the body of the planet, the rest of the surfaceappears diversified with dusky spots of an irregular character. Thesemaculae are generally of transient duration, hut some which appear in thevicinity of the belts are of a more permanent nature than the others ; andastronomers, by watching from time to time their position on the disk,have been enabled to establish the interesting fact that the planet revolvesround a fixed axis. It is worthy of remark, that the illustrious Kepler,previous to the invention of the telescope, had surmised that Jupiter revolves on an axis, and from a consideration of the rapid motion of thesatellites of the planet round their primary, compared with the slowermotion of the moon round the earth, he inferred that the period ofrotation is less than twenty-four hours. This sagacious conjecture hasreceived a remarkable confirmation from the researches of subsequentastronomers. Hooke appears to have been the first person who was ledby observation to suspect that Jupiter revolves on an axis. In the monthof May, 1064, he discovered a small spot on the largest of the threebelts. In the course of two hours, he found that it had moved from eastto west about half the length of the diameter of the planet-]-. With thatfickleness of purpose which characterized his conduct throughout life, heappears to have abandoned all further observations of the planet, leavingto others in this as in almost all similar instances, the glory of sub-stantiating the interesting fact which, by his superior acuteness, he wasenabled first to foreshadow. It was reserved for Cassini to prove, beyondall doubt, that Jupiter revolves on an axis, and to assign with considerableprecision the period of rotation. In the month of July, 1665, he observedthe planet with a telescope constructed by Campani, the object-glass ofwhich had a focal length of thirty-five French feet. He discovered agreat number of spots on the disk, some of which were brighter than therest of the surface, resembling in this respect the solar faculte. Theyall appeared to have a motion on the disk from east to west, but in everyinstance they exhibited such rapid variations of form and magnitude, thatno certain result could be deduced from them. At length he discovereda permanent spot situated close to the northern border of the mostsouthern of the three belts, which he continued to observe for severalmonths. It appeared largest on the centre of the disk, from which itgradually diminished until, before reaching the margin, it became invisible.Its motion also was swifter on the centre, of the disk than towards thecircumference. From these facts Cassini concluded that the spot adheredto the surface of the planet, and that the latter consequently revolves onan axis. With respect to the time in which it effected a complete

* Almag. Nov. tome i., p. 486. Dr. Moll, of Utrecht , in his interesting account ofthe invention of the telescope, published in the first volume of the Journal of the RoyalInstitution, ascribes the discovery of Jupiter ’s belts to Toricelli, the pupil of Galileo.

T Phil. Trans., No. 1. This communication, consisting of eight lines, is the secondwhichappears in the famous collection of the Transactions of the Royal Society .