20 REPORT— 1839. 



which, however, cannot he regarded as definitive until the per- 

 sonal equations of the Irisii observers shall have been compared 

 with those of Greenwich. 



The extreme consistency of the individual results, the great- 

 est differetice being 1^-65, is well calculated to inspire confi- 

 dence, and there is every reason to rest satisfied with these num- 

 bers, as the chronometric longitudes. 



Yet, however accurate they be, it is impossible to consider 

 the means by which they are obtained as superseding the me- 

 thod by signals. The first, transports the time from one station 

 to another by machines, which, though their performance is 

 wonderful, yet must be disturbed by that very process ; in the 

 second, the chronometer is light. Its application is far more 

 costly as well as difficult, but its certainty is greater, and the 

 whole of the disturbing causes are in view. The general cha- 

 racter of it is this, — The flame of powder at an intermediate sta- 

 tion is observed from the observatories, and the difference of the 

 times is that of longitudes. If, however, the interval is too great 

 for one signal, two with an intermediate observer are employed : 

 the eastern signal is observed by him and the eastern observa- 

 tory ; a short time after he observes the western signal in con- 

 junction with the west observatory, and the longitude is the 

 difference of observatory times, lessened by that which has 

 elapsed between the two signals. Thus any number of inter- 

 mediate stations may be employed. The powder is generally 

 fired on mountains, and it is found that the flash of small quan- 

 tities is visible at very great distances. Four ounces have been 

 seen at 140 miles. When mountains are not to be found, the 

 requisite height must be gained by rockets ; and an elegant ap- 

 plication of this is seen in Sir J. Herschel's operations for de- 

 termining the relative longitude of Greenwich and Paris, de- 

 tailed in the Philosophical Transactions for 1826. 



This kind of signals is essential in Ireland, and even with 

 them the local circumstances of Armagh are such as to present 

 great difficulties. A range of hills rises to the south, from 600 

 to 1000 feet above it, at about four miles distance, and these are 

 shut out from Dublin by high ground to the north of it. I was 

 deprived of the aid of Colonel Colby, by his absence in Scotland, 

 where he was engaged in making the necessary arrangements for 

 the completion of its survey, but my friend Lieut. Larcum sup- 

 plied all necessary information, guided by which the mountain 

 She-. e Gullion was selected. Its summit is visible from Dublin 

 at fifty-two miles, but is 800 feet below the land which bounds 

 my view, and this decided the size of the rockets necessary, as, be- 

 sides certainly clearing tliat ridge, they were to carry four ounces 



