The Rev. Dr. Robinson on the Longitude of the Armagh Observatory . Ill 



compression, but which in strictness ought to be investigated by independent 

 research. 



In cases when the apparent tract of the star is very oblique to the moon's 

 limb, its irregularities present a new source of error ; and the final result is, that 

 though the observations may be certain to a tenth of a second, the longitudes 

 deduced may differ several seconds, and the truth can only be attained by a mean 

 of many, taken under circumstances differing as much as possible. 



The method of transits of the moon and lunar stars, though it afford an easy 

 and pretty accurate approximation, is affected by the influence of irradiation, 

 which I believe to vary not only with the telescope, but also with atmospheric 

 changes. The personal equation is also different in some instances, for the 

 planet and the stars, as I infer from the fact, that the transits observed by my late 

 assistant gave the longitude five seconds of time less than those observed by my- 

 self after his death. In this method, therefore, it is necessary not merely to have 

 observations of each limb, but to multiply the stations of comparison, that among 

 the variety of observers and telescopes a kind of mean result may be obtained. 



The determination by chronometers depends on the perfection of these 

 machines, and in particular on their rate being unchanged by the agitation of a 

 long journey. This, strictly speaking, is never the case, though it is sometimes 

 very nearly accomplished, and its effect will disappear from the mean of the 

 results obtained in going and returning, if the circumstances of the two journies 

 are nearly similar. 



Unfortunately it rarely happens that an astronomer has the power of making 

 these experiments on a sufficient scale ; but such an opportunity seemed to Sir 

 William Hamilton and myself to present itself, in consequence of Mr. Dent's 

 chronoraetrlc visit to Paris, and the yet more remarkable notice, read at the 

 Newcastle Meeting of the British Association, of the Chronometric Longitude 

 of Sir Thomas Brisbane's Observatory. Mr. Dent not merely promised us every 

 assistance, but when, having obtained the consent of the authorities of our respec- 

 tive observatories, we proceeded to make the necessary pecuniary arrangements, 

 he treated the matter as one of science, not of commerce, and not only took on 

 himself the expense and risk of the journey, but came in person. 



The chronometers which he placed at our disposal were fifteen, of which 

 twelve were those that had been used in the determinations of Paris and 



