DETERMINING THE LONGITUDE. 239 



that can materially influence results. But this is not the case with respect to 

 the data from which eclipses of Jupiter's satellites are computed. The pre- 

 cision of the tables of that phmet, is by no means such as to dispel all doubt ; 

 the results deduced from observations of immersions and emersions, are liable 

 to be vitiated by many extrinsic circumstances, such as the state of the atmos- 

 phere, different powers of vision in the observers, and the uncertainty from 

 these and similar causes of the appearance and disappearance of tlie satellites. 

 In these remarks, I am fully borne out by the testimony of Mr. Francis Baily, 

 whose high attainments as an astronomer and mathematician, entitle his 

 opinion, on all subjects connected with practical astronomy, to the utmost 

 deference. But it is unnecessary to quote the sentiments of any particular 

 individual. There would appear then to be no question, as to the superiority 

 of lunar transits on the principle recommended by the Cape Astronomer, 

 The errors of the Lunar Tables must, in general, be very trifling; and in the 

 hands of a skilful observer, the transit instrument cannot partake of what is 

 usually termed instrumental error. But neither are the tables of Jupiter's 

 sateUites to be depended upon in the same degree, nor can the best and most 

 experienced observer control the circumstances, which should render his 

 observations free from the errors to which I have adverted. 



Having premised these observations, in illustration of the superiority of 

 lunar transits, I shall now proceed to explain, in as concise a manner as possi- 

 ble, the method of deducing the longitude from the transit of the moon's limb. 



It is presumed, that the reader has made himself acquainted with the se- 

 veral adjustments necessary to the portable transit instrument previous to its 

 being placed in the meridian. These adjustments consist, first, in setting the 

 vertical wires truly perpendicular to the horizon ; secondly, in rectifying the 

 parallax of the telescope and the line of collimation ; and lastly, in levelling 

 the axis of the instrument. 



