New Method of dekrminins; the Longitude. 109 



adopted, at one and the same tinne, bj persons in every 

 habitable part of the globe ; for it is applicable to situa- 

 tions distant H)0° in longitude from each other, and even 

 beyond that distance, as I shall show by one of the subse- 

 quent examples.'' p. b. 



The longitude of a place may be deduced from obser- 

 vations made on the culmination of the moon and stars by 

 calculations depending on the tables of the moon and stars ; 

 in which case, the correct time must be observed, and the 

 errors of the tables are involved. But Mr. Baily, in this 

 paper, speaks only of correspondini;; observations ; that is, 

 observations of the same bodies made at different obser- 

 vatories. In this case '' it will not be of any consequence 

 if the clock should not exactly show the correct time; as 

 it is the difference only, between two given moments, that is 

 required. All that is necessary therefore is, that the clock 

 should go correctly during the short interval of the tran- 

 sits. In fact, the whole method is a method of differences 

 only ; and it is to these differences that the principal at- 

 tention must be paid.'' • p. 11. 



This method of obtaining the longitude was first success- 

 fully, and not till recently, practised, by M. Nicolai, a 

 distinguished astronomer at Mariheim. He subsequently 

 published a list of stars that would be favourably situated 

 for corresponding observations of this kind, and invited the 

 co-operation of other observers. These stars he calls 

 moon- culminating stars — we wish he had chosen a better 

 term. The result of this proposal was, that successive 

 lists of such stars were published, " and already, at several 

 observatories, the observers have been enabled to deter- 

 mine their difference of meridian, in a few months, with as 

 much accuracy as they formerly could in as many years. 

 It might, indeed, at first sight, appear that the same re- 

 sults would be obtained, if we merely observed the correct 

 time of the moon's transit, without any reference to the 

 contiguous stars : but a moment's reflection will convince 

 us that, by referring the moon's border to the adjacent 

 stars, we obviate all errors net only of the clock, but also 

 in the position of the transit instrument." p 5. 



The last circumstance mentioned above, relating to the 

 position of the transit instrument, will be regarded as an 

 rmportant advautage possessed by this method ; especially 



