178 OF THE LATITUDE AND LONGITUDE.

that no use could be made of lunar observationsin discovering the longitude, either at sea orland. Sir Isaac Newton was the first who dis-covered the true cause of the inequalities in themoon’s motion, which before his time wereconsidered as inexplicable ; and the late ProfessorMayer of Gottingen, following the theory ofNewton, has formed a set of lunar tables, soaccurate and perfect, that the place of the moonmay be determined in any part of her orbit, towithin a minute of the truth at least.

But, besides the error here noted, severalothers will be apt to arise from the instrumentand the observations. The most experiencedobserver cannot be sure that he has obtained thetrue lunar distance to within less than half aminute of the truth at least, and this, joined tothe former one, will produce in the whole amistake of three minutes in time, which is equi-valent to three quarters of a degree of longitude.It will, however, conduce to a still greater de-gree of accuracy, if the moon’s distance be takenfrom two stars, or from the fun and a star oneach side of her, as often as opportunity per-mits j for as the imperfections of the instrument,as well as unavoidable small errors which attendthe use of it, have a natural tendency'to correcteach other, the mean result, arising from thesedifferent observation*, will generally be much

nearer