DETERMINING THE LONGITUDE. 237 



communication with him. The new method to which I alhide, is found- 

 ed on the principle of determining the right ascension of the moon's centre 

 from the observed transit of her limb, compared with that of the sun and one 

 or more stars, and then to find, from the Nautical Ephemeris, at what time 

 at Greenwich, the moon had the same right ascension. The accuracy of this 

 method depends partly on that of the catalogues of right ascension of such 

 Zodiacal or other stars, as may be used to compare with the moon's transit ; 

 hence only those stars should be used, whose right ascensions are accurately 

 determined. With a good clock or chronometer, therefore, adjusted to sidereal 

 or mean time, (the former is most convenient) and a transit instrument, placed 

 in the plane of the meridian, the longitude of any place may be determined 

 in two nights, within 15' of the truth. During the whole course of my 

 experience, the observations of any one night seldom differed more than this 

 quantity, from the mean result of the observations of several consecutive 

 nights ; and I may safely affirm, that with a good clock or chronometer, and 

 a transit instrument properly adjusted, one observation of the transit of the 

 moon, compared with two of the sun (when their declinations do not differ more 

 than 10°) one the noon preceding, and the other on the following noon, will 

 give the longitude probably within 10'. Some of the Greenwich stars, too, 

 whose apparent right ascensions are computed for every tenth day, will 

 always be visible on the meridian, and if the rate of the chronometer be 

 uniform, will, without further trouble, afford a more correct result, and one 

 more to be depended upon, than a series of observations of the eclipses of 

 Jupiter's satellites. 



With respect to errors, which may be supposed to arise from the imper- 

 fection of the Lunar Tables, I have already adverted to the pre-eminent ser- 

 vices rendered to practical astronomy, by the late Astronomer Royal. This 

 fact is, in no instance, more strikingly illustrated, than in the distinguished and 

 unequalled excellence of the Astronomical Ephemeris. The perfection to 



2 M 



