I HAVE been long importuned by several of my Friends to publish a Catalogue of Eclipses of Jupiter's Satellites, to the Intent that such Persons as are furnished with proper Instruments, and want neither Leisure nor Inclination to observe them, may no longer neglect the frequent Opportunities that offer, for want of timely Notice.

The great Number of Eclipses that happen in a Year, as appears by the following Catalogue, amounting to 352; notwithstanding the fourth Satellite will pass wide of the Shadow, after the middle of January next; the Ease with which they are observed, especially since the great Improvement made to the reflecting Telescope by Mr. Hadley; the little Skill that is required to make the Observations, since the Difference of Times, when observed by the largest: Glasses, and the smallest through which they may be seen, amounts to scarce one quarter of a Minute; render these Observations the most proper of any that the Heavens afford us, at present, for determining the Longitude of Places; and I may venture to assert, that there are very few Places of Note upon the Surface of our habitable Globe, whose Longitudes are already known, that have not either been absolutely deter-