DETEHMINING THE LONGITUDE. 251 



And, making allowances for errors of observation, it is evident that the in- 

 strument was well adjusted. The method of adjusting a transit instrument 

 by circumpolar stars, is seldom or ever practicable in low latitudes. I have 

 already stated, that in India the Pole Star is best adapted for this purpose, and 

 the adjustment may be verified by computing from the equatorial interval 

 between the wires, the instant that Polaris should bisect the 1st, 2d, and 3d 

 wires. 



I beg to remark, in conclusion, that I have deemed it unnecessary to in» 

 troduce into this Memoir, analytical formulae of demonstration, which often 

 perplex the student, who is not well grounded in Mathematics, and which, 

 perhaps, not unfrequently deter him from pursuits in Practical Astronomy 

 and Geography. My object is, to render easy and intelligible, what may be 

 accounted difficult by those who know little more than the rules and prac- 

 tice of Nautical Astronomy, and who yet may have excellent opportunities 

 of multiplying Observations, for the improvement of Geography ; — in short, 

 to recommend the more general use of the Transit Instrument, in preference 

 to any other method for determining the longitude on land. 



But, although I am decidedly of opinion, that the method of lunar 

 transits here explained, is to be preferred to eclipses of Jupiter's satellites, 

 yet I would not be understood to invalidate the utility and expediency of 

 embracing every opportunity that oifers, of observing them. 



The results deduced from corresponding observations of immersions and 

 emersions, made under different meridians, I consider to be unexceptionable ; 

 while those deduced from a comparison, with the computed times given in 

 the Ephemeris, clearly show, that the Tables of Jupiter, have not yet attain- 

 to that degree of perfection, which is indispensably necessary to ensure gene- 

 ral confidence. 



