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inconsiderable in magnitude corresponded at great distances. 
If such should prove to be the fact, it would be only neces- 
sary to project ona large scale the results of the observations 
made about the same absolute time at the two stations, and 
to compare the times of the corresponding maxima and 
minima. In the observations already referred to, the maxima 
and minima succeeded each other (as has been said) at inter- 
vals of about forty seconds, and the epoch of their occurrence 
was probably known to six or seven seconds. By shorten- 
ing still further the interval of observation, it is manifest 
that this error may be much diminished. The corresponding 
error of the difference of longitude resulting from a single 
comparison (supposing the probable error of epoch to be the 
same at the two places,) will be greater in the ratio of 2 
to 1; but this error, owing to the multitude of the maxima 
and minima compared, must necessarily be greatly reduced 
in the final mean. 
In order to put this question to the test on the largest 
scale, it was agreed between Mr. Bache and Mr. Lloyd, to 
make a series of corresponding observations in Philadelphia 
and in Dublin. Some difficulties occurred in concerting a 
plan, and Prof. Bache underwent, in one instance, the labour 
of an extensive series of observations, without any counter- 
part in Dublin. At length, however, it was agreed to ob- 
serve during the week commencing the 11th of November, 
1839; the observations being taken during two hours on 
each day—namely, from 12 to 1 p.m., and from 8 to 9 p.m., 
Greenwich mean time. 
Prof. Bache’s account of his observations is contained in 
the following extract of a letter which accompanied them, 
dated November 29, 1839. y 
** The place of observation is a room in one of the 
out buildings for the dwellings of the Professors of the 
Girard College. As the materials used in the construc- 
tion of the house must produce considerable local attrac- 
