446 On dclermining the Longitude 



pleted by one ohserver; and if he has a good eye, and is 

 careful in making his different sets of observations, it is 

 more than probable that the whole will be conducted vviih 

 much greater accuracy than when three separate observers 

 are employed, as it seldom happens that each of the ob- 

 servers is equally correct and careful ; and should it be 

 discovered that the one who takes the sun's altitude is 

 inaccurate, the whole deduction will be affected with that 

 inaccuracy : nor indeed is it scarcely possible for each Qb- 

 server taking the altitudes to complete his observation at 

 the same time that the one who takes the distance has 

 completed his; and the inaccuracy arising will be of con- 

 siderable importance, should this happen to the observer 

 who t)bserves the object from vyhence the time is to be 

 computed. 



In the method I have adopted, the only inaccuracy af- 

 fecting the observation arises from the altitudes of the 

 bodies not increasing or decreasing uniformly: this, in the 

 sun or a star, is of no importance if the observer is at all 

 expert at making his observation, and in the moon the 

 error will be too small generally to deserve notice. The 

 watch by which the observations are made, should have a 

 seconds hand, and ought not to vary more than one minute 

 in twenty-four hours; — a good horizontal or duplex watch 

 will answer very well, and will allov.' time for the observer 

 to take his final set of observations for determining the 

 error of the watch without hurrying himself. 



The deductions derived from a mean of the four sets of 

 observations about to be enumerated, give the longitude of 

 the place in time 38,9 sec. west of Greenwich ; or in space 

 9' 3l",35 west*. By a reference to the best maps of 

 London, it appears that the Polygon, Somers Town, is 

 about 400 yards to the east of St. James's Church, Picca- 

 dilly, and the k)ngilude of this church is according to the 

 trigonometrical survey, 8' 3" west of Greenwich. The 

 observations gi 'e the longitude of the Polygon, Somers 

 Town, l' 26", 35 to the west of St. James's Church, or 

 about one mile ; this added to the distance that the Polygon 

 is to the east of St. James's Church, makes the whole error 

 in longitude deduced from these observations about one 

 mile and a quarter, which, taking into consideration the 

 errors of observation, of the instruments, and of the lunar 

 tables, will be found very small ; and I am fully persuaded 



* The obscrvatioDS were takeu- near the Polygon. 



that 



