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568 PHIPPS’s JOURNAL.
The diftance between the two fhips, deduced from the altitude of the
matt, was - - 2457 feet.
By the angle of the main and main topfail-yard, the diftance between
them being 34,125 feet, - - 2640 feet.
Difference 183 feet.
Which is not more than the fhips might have changed their pofition in the time of
reading off and fetting down the firft obfervation before taking the fecond.
An error of ten feconds in the obfervation of the angle fubtended by the maft at this
diftance, would make an error of two feet and three quarters in the diflance. At the
diftance of a nautical mile it would produce an error of fixteen feet. At other dif-
tances the error decreafes as the fquares of the diftances decreafe ; and at other heights
it decreafes as the heights decreafe.
Whenever the diftance of the objet, whofe angle is taken by the megameter, does
not exceed that of the vifible horizon, the very {mall portion of the earth’s furface in-
tercepted between the object and obferver, may be confidered as a plane, to which the
objeét is perpendicular, and the diftance may be concluded by refolving the right-
angled triangle, formed by the upright object, and lines drawn from the obferver’s
{tation to the top and bottom of it.
Obfervations on the Variation.
Tue variation of the compafs, always an interefting objec to navigators and phi-
lofophers, became peculiarly fo in this voyage from the near approach to the pole.
Many of the theories that had been propofed on this fubjeé&t, were to be brought to
the teft of obfervation made in high latitudes, by which alone their fallacy or utility
could be difcovered. ‘They of courfe engaged much of my attention, and gave me
the fulleft opportunity of experiencing, with regret, the many imperfections of what is
called the Azimuth compafs. This inftrument, though fufficiently accurate to enable
us to obferve the variations fo as to fteer the fhip without any material error, with the
precaution of always ufing the fame compafs by which they are taken, is far from being
of fuch a conftruétion as to give the variation with that degree of precifion, which
fhould attend experiments on which a theory is to be founded, or by which it is to be
tried. he obfervations taken in this voyage will fully evince this by their great va-
riations from one another in very fhort intervals of time; nor is this difagreement of
fucceflive obfervations peculiar to the higher latitudes, and to be imputed to a near ap-
proach to the pole, as I found it to take place even upon the Englith coaft.
As to the obfervations themfelves, they were taken with the greateft care, and the
moft ferupulous attention to remove every circumftance which might be fuppofed to
create an accidental error; the obfervations being taken fometimes by different people
with the fame compafs, in the fame and different places; fometimes with different
compaffes, changing the places and the obfervers repeatedly, to try whether there was
error to be imputed to local attraction, or the different mode of obfervation by different
perfons. 1 have fince my return-tried the compafles by a meridian as well as by taking
azimuths, and find them to agree with one another, though the fame compafs fome-
times differs from itfelf a degree in fucceflive obfervations.
That every perfon may (as far as is poflible without having been prefent at the time)
be enabled to judge of the degree of accuracy to be expected in fuch obfervations, as
well
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