136 The Rev. Dr. Robinson on the Difference of Longitude 



As the most important part of longitude measures is the determination of 

 the Observatory time, I annex the transit observations, and the clock corrections 

 deduced from them. 



The instrument at Armagh is 5:^ feet focal length, and 3.8 inches aperture, 

 power 1 60 ; its axis was examined by the level daily, and its meridional position 

 constantly verified by two marks, which being exactly adjusted to the meridian, 

 would also detect any error of coUlmatlon, if it existed. This was insensible, as 

 also is shown by six reversions made on May 25th, for the purpose of verifying 

 the equality of the pivots, the difference of which is given by them = 0'. 0004, 

 in fact, evanescent. At the same time their figure was tried by examining the 

 inclination at every twenty degrees from the northern to the southern horizon ; 

 but though tenths of seconds of space can be estimated on the level, no error 

 could be found. The transits were, except in two instances, taken by Mr. 

 Edmondson. 



At Dublin, they were taken by Mr. Thompson : the instrument has six feet 

 focal length, and four inches aperture, power = 100. The inclination of its 

 axis was found by the level, on the 8th, 17th, 22nd, and 23rd, = -}-2'. 18; its 

 meridional position by nine observations of Polaris, from April 30 to May 22, 

 and its error of coUimation by four of the same star, on May 20th, reversing 

 between the wires, from which it appears that the observed transits require tlie 

 correction, 



— o'. 5371 + o'. 6134 tang 8 — o'. 1059 secant 8. 



The clock corrections are deduced from the places of Encke's Jahrbuch, 

 which for 7 Ursa?, and some other stars, agree better with our observations than 

 those of the Nautical Almanac. 



