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differences of longitudes by signals, geodetic measures, oc- 
cultations, and transits, the author passes to the most direct 
and natural method, by transport of chronometers. He 
says, ‘‘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 journeys 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 chronometric visit to Paris, and the yet more re- 
markable notice, read at the Newcastle Meeting of the 
British Association, of the chronometric longitude of Sir 
Thomas Brisbane’s Observatory. Mr. Dent not merely pro- 
mised us every assistance, but when, having obtained the 
consent of the authorities of our respective observatories, 
we proceeded to make the necessary pecuniary arrange- 
ments, he treated the matter as one of science, not of com- 
merce, and not only took on himself the expense and risk of 
the journey, but came in person.” 
Dr. Robinson proceeds to mention the particulars of the 
journey, and of the comparisons which were made with the 
fifteen chronometers which Mr. Dent brought with him. 
Their rates and errors, as compared with Greenwich time, 
had been determined, before starting, for the epoch of the 
20th of September, 1838; they were compared with Dublin 
time, in the observatory of Trinity College, on the 22nd of 
that month, and with Armagh time on the 23rd and 24th; were 
again examined in Dublin, on the 25th, and in Greenwich, 
on the 27th of September. 
Y 
