s, there are eighty-seven, or nearly one-fourth, which have the
intervals between all the wires agreeing to the same, the tenth of
a second. At Sierra Leone, nineteen out of seventy-two have the same
accordance; and of the moon culminating stars, p. 409, twelve out of
twenty-four are equally exact. With larger instruments, and in great
observatories, this is not always the case.

Captain Kater has given, in the Philosophical Transactions, 1819, p.
427, a series of transits, with a three and a half foot transit, in
which about one-eleventh part of them only have this degree of accuracy;
and it should be observed that not merely the instrument, but the stars
selected, have, in this instance, an advantage over Captain Sabine's.

The transit of M. Bessel is five feet in length, made by Frauenhofer,
and the magnifying power employed is 182; yet, out of some observations
of his in January, 1826, only one-eleventh have this degree of
accordance. In thirty-three of the Greenwich observations of January,
1828, fifteen have this agreement, or five-elevenths; but this is with
a ten-feet transit. Now in none of these instances do the times agree
within a tenth of a second between all the wires; but I have accounted
those as agreeing in all the wires in which there is not more than
four-tenths of a second between the greatest and least.

This superior accuracy of the small instrument requires some
explanation. One which has been suggested is, that Captain Sabine
employs a chronometer to observe transits with; and that since it beats
five times in two seconds, each beat will give four-tenths of a second;
and this being the smallest quantity registered, the agreement becomes
more probable than if tenths were the smallest quantities noticed. In
general, the larger the lowest unity employed the greater will be the
apparent agreement amongst the differences. Thus, if, in the transit of
stars near the pole, the times of passing the wires were only registered
to the nearest minute, the intervals would almost certainly be equal.
There is another circumstance, about which there is some difficulty. It
is understood that the same instrument,--the thirty-inch transit, was
employed by Lieutenant Foster; and it has not been stated that the wires
were changed, although this has most probably been the case. Now, in the
transits which the later observer has given, he has found it necessary
to correct for a considerable inequality between the first