LET. X. OF THE LATITUDE AND LONGITUDE. 1 77

yet to a person of skill and abilities, the timeand labour it requires are the principal objec-tions against it. But as the success of any me-thod, must depend upon the accuracy withwhich it can be practised, it may not be amissto give you some idea of the degree of precisionthat we may hope to obtain in finding the lon-gitude from lunar observations; or, which isthe fame thing, by taking the distance betweenthe fun and moon, or between the moon andsome noted fixed star.

In the first place then it may be observed, thatthe moon’s daily motion in her orbit being aboutthirteen degrees, her hourly mean motion isabout half a degree, or one minute of a degreein two minutes of time; so that if an error ofone minute be committed in calculating the placeof the moon, it will produce an error of twominutes in time, or half a degree of longitude.If the motion of the moon was more rapid thanit is, it would afford us the means of attaininga still greater degree of perfection ; and, on thecontrary, if her motion was flower, so that wecould scarcely discern her change of place fromone day to another, we could receive no advan-tage from it in discovering the longitude.

It was not till within half a century past, thatastronomers were able to calculate the place ofthe moon to within less than six or seven minutes°f the truth; and this error was so considerable,

N that