1840.] Fort William, and Fultehghur. 81 



tendency of Mr. Taylor's subsequent operations at Madras, 

 that the meridian of Fort William Flag-Staff will prove to be 

 less than the above. 



This method of determining longitudes, or rather differences 

 of longitude, has been much recommended of late by astrono- 

 mers ; and doubtless it is as capable of great accuracy, when a 

 long series of corresponding sights can be taken in fixed obser- 

 vatories 5 but to those to whose lot it falls, for the most part, 

 to determine new positions, — to military and maritime officers, 

 and to scientific travellers, — it will not I fear be found so ge- 

 nerally convenient, as it may appear to be. It is requisite, that 

 the transit instrument be good, and well and firmly fixed, and 

 that the sights be most carefully made, for an error of only two- 

 tenths of a second of time, on the observation of the transit of 

 the moon's limb, will on her mean motion cause an error of six 

 seconds of time in the longitude. To duly estimate a small 

 part of time requires much practice, and it is difficult to be sure 

 of the precise instant when the moon's preceding limb touches 

 the wires, it is perhaps rather less so of the following limb lea- 

 ving the wire, but a mean must be taken ; add to this, that 

 except to those, whose sole occupation is in a fixed observa- 

 tory, it would be very irksome to get through a long series 

 of lunar transits, at the varying periods of three quarters of 

 an hour's difference of time, every night. On these accounts, 

 I hope the satellites of Jupiter (especially the first) will meet 

 with more favour than has been allowed to them lately, in some 

 notices on practical astronomy. I believe that by their means, 

 the meridian of more distant places have been nearly settled, and 

 more useful additions, in that particular, made to Geography, 

 than by any other mode ; and from long experience, I find that 

 great dependence is to be placed on the results, provided the 

 requisite conditions, which I have mentioned, are attended to. 

 In this extensive country, we little need insist on the import- 

 ant consequences of well determined differences of longitude. 

 Moderate distances, can be best laid down from survey, and 

 referred to some known meridian ; but it frequently happens in 

 the emergency of service, that officers even on a survey, are 

 detached to a great distance from their field of operations, with 



M 



