of the Cambridge Observatory. 169 



This determination differs from that which we have just 

 found, not less than l',06, or 16" of space. This is an enormous 

 discrepance. An error of 1' could not exist in any obsei-vatory : 

 and an error of 16" in a survey would imply a linear error of 

 nearly 300 yards. Some other cause must be sought to explain 

 this disagreement, and I see but two which can with any pro- 

 bability be brought forward. One is, an error in the pi-ocess, by 

 which the difference of longitudes has been determined : the other 

 is, an irregularity in the Earth's form, or a sensible local at- 

 traction. 



With regard to the first of these, I am well convinced that 

 the error of the clock at Greenwich and at Cambridge was known 

 within one-tenth of a second. On taking the mean of several 

 transits, the mere error of observation is insensible : in the quan- 

 tity which I have mentioned I think that I have made sufficient 

 allowance for the errors in the position of the instrument, &c. 

 The rate of the Cambridge clock was pretty steady : that of the 

 Greenwich clock had altered on Oct. 20, but it appears to have 

 been steady during these observations. The constant difference 

 in the mode of observation of different observers would give a small 

 quantity, which might be added to the former. As to the possible 

 error in the comparison of the clocks, the Society can judge 

 from the details laid before them whether an error can be sup- 

 posed sufficiently great to reconcile the two determinations. My 

 own opinion is that it cannot exceed two or three tenths of a 

 second. On the whole, it appears to me that by the most violent 

 and improbable combination of possible errors, not more than half 

 the difference can be accounted for. 



I am forced therefore to recur to the other cause, and to hold 

 the opinion that the hypothesis of perfect regularity in the Earth's 

 figure is erroneous to an amount far greater than the probable 

 errors of observation. 



Vol. III. Purl I. Y 



