144 Report of Observations on the Transit of Mercury. 
tact could be recognized with entire confidence until at least 
twenty seconds after it had actually taken place. The Cincin- 
nati observations give the time of first contact within thirty one 
seconds of the computed time; and those of Hudson, Ohio, dif- 
fer from the time assigned in the American Almanac, by only 
about four seconds,—a coincidence which would seem to confirm 
the computation in a remarkable degree, did not the very exact- 
ness imply that the computation was too late, since the ingress 
could not have been distinctly seen so soon after the moment of 
contact. The West Point observations give the time of ingress, 
(marked “uncertain’’) 1m. 28-7s. later than the time assigned in 
the United States Almanac; and the Charleston observations give 
the time of ingress 2m. later than that assigned by the same 
authority. The time of first contact at Washington, as reported 
by Lieut Maynard, was 17-1s. earlier than the computed time, 
being (when corrected for error of clock) 11h. 11m. 1-9s., while 
the time given in the United States Almanac was L1h. 11m. 19s. 
If we compare the intervals between the first and second con- 
tacts, a like or even greater discrepancy will appear. 
m. §&. 
At New York, this interval was, 2 38* 
At West Point, ge Me 3 3 31:2 
At Western Reserve College, ‘ re a 3 34:3 
At Cincinnati, if i HH 3333 
At Charleston, Ke ff ‘ Sidi 
These comparisons show that little reliance can be placed 
upon the observations made on the first internal contact in a 
transit of Mercury. Similar discrepancies have also been re- 
corded by accomplished observers abroad. ‘Thus in the ob- 
servations made at Utrecht of the transit of May 5, 1832, by Dr. 
Moll and his assistants, the difference between observation and 
computation varied, as reported by the different obervers at the 
same place, nearly a minute.t Beside the inherent difficulties 
arising from the slow relative motion of the planet, and the want 
of perceptible indentation until it has advanced far on the sun’s 
limb, the magnifying and defining powers of the telescope will 
also have much influence. This is especially the case with the 
observations on the internal contacts, where the visibility of the 
* The time of actual ingress was not given, but only the time when first seen. 
t Ast. Trans, vi, 114. 
