168 Professor Airy on the Longitude 



These observations I consider equivalent to a greater number 

 of observations at the Royal Observatory, as the errors of level, 

 collimation, and deviation from the meridian, were well known, 

 and the necessary correction had been applied, according to the 

 system of the Cambridge Observatory. 



The clock's rate as found by Arcturus was + 0,36 : by a Aquilae + 0,36 : 

 by a Cygni + 0,39 : by the next observation' of Arcturus + 0,48 : the mean 

 is +0,39. 



Using this mean rate, and the mean clock error at the mean 

 time just found, we get these errors, 



Oct. 21, Cambridge clock at 5 . 49, the Cambridge clock was fasti ^ 



on Cambridge sidereal timej 

 Oct. 22 5.23 2 . 2,18. 



Compai-ing these errors of the clock on Cambridge sidereal 

 time with its errors on Greenwich sidereal time as already found, 

 it appears that Cambridge sidereal time is faster than Greenwich 

 sidereal time, 



by the first comparison 23',63, 



by the second 23,45. 



The mean of these cannot be far from the truth : and I think, 

 therefore, that we may for the present safely state the longitude 

 of the Cambridge Observatory to be 23',54 East of the Royal 

 Observatory at Greenwich. 



In the Trigonometrical Survey of England, the steeple of 

 Grantchester church was observed at the two principal stations 

 of Orwell and Madingley, and its longitude was thus determined 

 to be 6'. 9" East of Greenwich. The meridian mark of the Cam- 

 bridge Transit Instrument is on that steeple, and consequently 

 the longitude of the Transit Room, as determined geodetically, 

 is 6.9", or 24',6 of time East of Greenwich. 



