126 The Rev. Dr. Robinson on the Di^erence of Longitude 



showed at once that our choice lay between two — Loughanleagh, in the county 

 of Cavan, and Slieve GuUion, at the southern extremity of Armagh. The 

 first would divide the distance better, but as its line passes through the smoke 

 of the town of Armagh, the other was adopted. 



Its summit, 1893 feet above the sea, is occasionally visible at Dublin, but is 

 800 feet below my view, the distances being 50.9 and 18.2 miles, as shewn in 

 the annexed map, for which I am obliged to Lieutenant Larcom ; the section 

 beneath shows the character of the intervening land. From this, the necessary 

 size of rockets can be inferred; the pound rocket (1'". 7 diameter) rises 1400 

 feet, on an average, but cannot carry four ounces of powder, while it is evident 

 from Sir J. Herschel's paper, that the two-pounder (2'". 1 diameter) is quite 

 sufficient. These projectiles, when of such a size, require extreme care in the 

 details of manufacture ; and, if ill made, are not merely uncertain, but actually 

 dangerous ; and the case seeming of sufficient importance to authorize an appli- 

 cation to Government, I made an application to the Board of Ordnance, stating 

 the nature of my work, and requesting a supply of rockets. My reliance on 

 that liberality which I have always found in the Government, when the import- 

 ance of any scientific object is duly laid before them, was not disappointed, and 

 I have much pleasure in acknowledging the kindness with which the Master- 

 General, Sir Hussey Vivian, and the other members of the Board attended to 

 me ; not merely giving the rockets, but tents for the firing party, and other 

 matters which were necessary, but which I had in the first instance overlooked.* 

 I may add, that as a measure of precaution against the interference of curious 

 visitors, two of the police were placed at my disposal ; it was, however, un- 

 necessary, as, though great crowds of the peasantry were attracted by an 

 exhibition so new to them, they shewed every disposition to oblige and assist. 

 Having made all requisite preparations, I proceeded, on the 13th of May, to 



the theodolite, the angle between the telescope and the pile on the mountain top, where our rockets 

 were to be fired ; it proved 180°. 0'. 0"., or the three points were in one right line. 



* The rockets were remarkably good ; not one burst, which certainly is a singular contrast to the 

 French rockets in Sir J. Herschel's and Colonel Bonne's operations. Their average rise, on the 

 only evening that I measured it, was 800 yards ; they had, however, only four ounces of powder, 

 but the part of the case which contained it weighed six ounces more, so that they actually carried a 

 greater weight than the French. 



