258 DETERMINATION OP THE liONGITUDE OF SEYERAL, 



The value of i for the Greenwich transit instrument, viz. -t- 0-2 sec. 

 in time, or + 3" in space, is given by the Astronomer Royal, and is 

 found to agree precisely with that which he derives from the mural 

 circles, from similar observations of the vertical diameter of the moon, 

 corrected for the defective illumination. This coincidence would 

 seem to show that this correction is required by the actual dimensions 

 of the moon, and that if other transit instruments indicate a different 

 correction, it must be from inferior optical capacity. 



For the other instruments, with which Captain Talcott's must be 

 compared, I have computed, by the above formulae, the requisite cor- 

 rection of Burckhardt's semidiameter, as far as it could be derived from 

 all the observations of the transit of both limbs of the moon, on the 

 same day, which I have been able to find. The correction for the 

 mean of all the results is -f- 0-15 sec. in time, or 2 "-25, in space. The 

 separate results are given in the following Table, 



