Mr. Henderson on the difference of meridians, &c. 287 
has occurred in calculating the combined observations of the 
same day, the gain of mean on sidereal time being stated to 
be — 4 9 *54 (pp. 120 and 122), in place of — 4 S '24- On 
checking the other observations, a few trifling alterations 
appear to be necessary upon the Greenwich Table of side- 
real time, from the data given along with it. These seem 
to be occasioned by different methods of calculation, and 
indeed are hardly worthy of notice. The French astrono- 
mers not having given the data on which the calculations of 
the sidereal times at Paris are founded, they are assumed to 
be correct. 
The effect of the alterations thus made upon the elements 
of computation, is to redeem the result of the observations 
of 21st July from the suspicion which attached to it, in con- 
sequence of its disagreement with the results of the other days, 
and to produce a change of one-tenth of a second (correspond- 
ing to one hundred feet nearly on the earth’s surface, ) upon 
the most probable value of the difference of meridians forming 
the subject of investigation, it now appearing to be 9“ 2i s *5. 
At the same time the chronometer, which was employed for 
the observations at Fairlight down, is shown to have kept a 
more uniform rate than what previously appeared. In this 
important national operation, the utmost accuracy is desira- 
ble; and it has therefore been thought proper to subject the 
whole observations to a new computation. This will make 
more apparent the near coincidence of the results of the 
different days observations, and the great precision to be 
expected from experiments of the nature of those in ques- 
tion, when, as has happened in the present instance, they are 
