112 The Rev. Dr. Robinson on the Longitude of the Armagh Observatory^ 



Makerstown. These, latter were rated for some days at the Royal Observatory^ 

 Greenwich, and on September 20th were delivered to Mr. Dent. The remain- 

 ing three were timed by the pupils of the Marine School at Greenwich, on 

 the same day. They were packed in two boxes, and kept steady by a stuffing 

 of horse hair, which to me at least appeared a very insufficient guard against the 

 concussions of their rapid journey, but it seems to have been effectual. Much of 

 this journey was performed with the marvellous rapidity of modern improvement, 

 yet it may be questioned whether a slower passage would not have been more 

 favourable ; for the jarring of the railroad is severe, and the peculiar vibration 

 of a steam-vessel I know to be very liable to disturb the performance of a chro- 

 nometer. In this instance, of the total distance travelled, 275 miles were sea, 

 190 in Ireland in the common cars or stage-coaches, and the rest, amounting 

 to 500, were performed on railways. 



On the morning of September 22, the watches were compared at the Dublin 

 Observatory, with the transit clock, by Sir William Hamilton, his assistant Mr. 

 Thomson, and Mr. Dent himself; and on that of the following day, at Armagh, 

 by Mr. Dent and myself. As Mr. Dent's time was precious, and I attach little 

 or no importance to stationary rates, he started on the evening of the 24th, after 

 we had again each compared the watches ; and revisiting Dublin on the follow- 

 ing day, and again making the comparisons, he sailed in the evening for 

 Liverpool. The watches were finally returned to Greenwich, and compared by 

 Mr. Main with the transit clock on the 27th, shortly after noon. 



In making these comparisons, the Dublin astronomers appear to have taken 

 beats of the watches, and divided the seconds of the sidereal clock. Mr. Dent 

 took beats of the clock, and divided those of the watch, and I waited for coinci- 

 dences and separation of the beats, — far the most accurate, but also far the most 

 tedious mode of comparison. My results were, however, almost identical with 

 Mr. Dent's. 



Mr. Main, I believe, used the same method ; for entire and half seconds only 

 appear in his comparison, as must be the case when the watches beat twice in the 

 second. 



If we denote by e the correction of a watch when leaving the eastern, w that 

 when arriving at the western station, i the interval of the watch's time between 



