160 Professor Airy on the Longitude 



These observations will explain the reasons which induced 

 me to take the first opportunity of determining astronomically 

 the longitude of the Cambridge Observatory, though it had 

 already been determined geodetically. And the remarks on the 

 inconvenience of some of the astronomical methods, and the in- 

 accuracy of others, will account for the preference of the method 

 used in the present instance. During my residence at the Ob- 

 servatory, with all the care that could be bestowed by a single 

 observer (unassisted indeed, and fully employed with other ob- 

 servations), only one occultation, and one eclipse of Jupiter's se- 

 cond satellite have been observed. The corresponding observation 

 of the former was not made at Greenwich,- and any result of the 

 latter would be doubtful to the extent of 10' at least. These 

 methods, therefore, have hitherto been ineffectual. The method 

 of artificial signals, as I have stated, is too troublesome. A series. 

 of lunar transits has been observed, but this observation must be 

 repeated long before a good result can be obtained. The only 

 practicable method remaining was that of transporting chro- 

 nometers ; a method which has scarcely been used on land, but 

 which tlie facility of communication between Cambridge and 

 London seemed to make in this instance equal or superior to 

 all the others. 



An opportunity for trying this occurred in October last. Six 

 chronometers had been lent by the government to Professor 

 Whewell, Mr. Sheepshanks, and myself, for the prosecution of 

 some experiments on the intensity of gravity in the deep Cornish 

 mines. The trial of steadiness to which they were subjected was 

 severe, and they sustained it well. These chronometers were still 

 in our possession on my return to the Observatory: and I sug- 

 gested to Mr. Sheepshanks, who was then residing in London, 

 and of whose co-operation I had no doubt, that before returning 

 them to the Koyal Observatory, they might be usefully employed 



