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IV. On the difference of Longitude between the Observatories of Armagh and 

 Dublin, determined by Rocket Signals. By the Rev. T. R. Robinson, D.D., 

 Member of the Royal Irish Academy, and other Philosophical Societies. 



Read 24th June, 1839. 



In the communication respecting the Chronometric Longitudes of Armagh 

 and Dublin, which I had the honor of submitting to the Academy last winter, I 

 mentioned that it was our intention to determine the difference of our meridians 

 by rocket signals ; this has since been performed, and has given results which 

 are the subject of this paper. 



The method of signals is the most obvious of all, and under favourable cir- 

 cumstances, the most accurate. In it, the time of one place is transported to 

 another, not by any machine, imperfect in its performance, and disturbed by 

 that very transporting ; the chronometer in it is light. If the appearance used 

 for a signal be instantaneous, the only known source of error is in the deter- 

 mination of the Observatory time, which equally affects all other longitude 

 methods. It appears to have been first used by the celebrated Picart, in a 

 journey to Denmark, for the purpose of ascertaining the true position of Tycho's 

 Observatory. He caused a fire to be kindled on the tower of the Observatory of 

 Copenhagen, which was occasionally covered by a screen, and the time of its 

 disappearance noted there, as well as by an observer at the ruins of Uraniburg. 

 The distance is not more than seventeen miles, and there must have been some 

 difficulty in covering the fire rapidly, as, from a passage in another of Picart's 

 works, it appears to have been three feet in diameter. If, instead of a fire, one 

 of Drummond's lights, placed in the focus of two Fresnel's lenses, directed to 

 the stations, were suddenly covered by a hood, we should have a signal visible 

 at any distance ; which, besides being perfect in its nature, might serve to 

 remove a doubt which has sometimes occiured to me. The impression of a 



VOL. XIX. R 



