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PHIPPS’s JOURNAL. sox
«* The watch was compared every day about noon with the two time-keepers made
by Meff. Arnold and Kendal; and from this comparifon, and their rates of going pre-
vioufly fettled at Greenwich, together with knowing, how much they differed from mean
time at Greenwich before we fet out, was calculated the table which fhews what the
mean time is at Greenwich according to each time-keeper, when the watch is at twelve
hours.
“< By the help of this table, we may eafily find the longitude of the fhip, as deduced
from the going of each time-keeper. Having found how much the watch is too faft |
or too flow for mean time at the fhip, we know what the mean time is at the fhip
when the watch is at twelve hours; and by the table we can find what is the mean time
at Greenwich at the fame time, fuppofing each time-keeper had kept at the fame rate of
going as it had before our departure: the difference of thefe mean times will give the
longitude of the fhip.
«* For example, June roth, in the afternoon, the watch was 1’ 24” too flow for
mean time at the place where we obferved; therefore, when the watch fhews twelve
hours, the mean time at this place was 12" 1/24”. At this time I find by the table,
that, according to Kendal’s time-keeper, the mean time at Greenwich was 12" 2/ 7’:
from this fubtracting 12° 1’ 24”, the mean time at the fhip, the remainder, o’ 43” is
the difference of meridians ; which, converted into parts of a degree, gives 0° 10/ 45”
for the longitude of the fhip according to Kendal, which is to the weltward, becaufe
the mean time at the fhip is lefs than that at Greenwich.
*¢ When we were on fhore, the obfervations were made with an aftronomical
quadrant, divided by Mr. Ramfden, of eighteen inches radius, which was placed ona
folid rock of marble; the error of the line of collimation was found by inverting the
quadrant, which was adjufted by a fpirit level. The weather did not permit us to take
correfponding altitudes of the fun, fo that we determined the apparent time by com-
putation from altitudes of the fun’s lim’); having before fettled the latitude of the
place of obfervation, from meridian altitudes of the Sun’s limbs taken with the fame
inftrument.
“¢ The latitudes of the fhip were uctermined moft commonly by the meridian alti-
tude of the fun’s lower limb; in a few inftances, by that of his upper limb, when the
lower was not fo diftin&t, or was hid by the clouds. ‘The height of the eye above the
level of the fea, in all thefe obfervations, was fixteen feet. When we could not get a
meridian obfervation, we. made ufe of the method defcribed in the Nautical Almanac
for 1771, from two altitudes taken about noon, and at a little diftance from it.
** It fometimes happens that we can only take fome altitudes very near the time of
noon. If we have obferved any altitudes of the fun nearthe prime vertical, we may
thence determine how much the watch is too faft or too flow for apparent time; and
confequently, how mnch the time when the altitudes were taken, is diftant from
noon; it therefore remains how to find how much thefe altitudes are different from the
meridian altitude. ‘This may eafily be found by the following rule :
“ To the logarithm of the rifing, taken out of the tables in Nautical Almanac for
1771, add the complement arithmetical of the logarithmic cofine of the fuppofed meri-
dian altitude ; from the fum (the index being increafed by five) fubtrac&t the logarithm
ratio (found by the rules in the abovementioned Ephemeris) the remainder is the log-
arithmic fine of the change in altitude.
sé ExaMPLz
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