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566 PHIPPS’s JOURNAL.
five feet to thirty feconds, four miles ahead of the fhip. On the paffage home, the log at
fi ty-one feet to thirty feconds, thirty-five miles aftern of the fhip ; at forty-five to thirty
feconds, one degree feven minutes ahead of the fhip. As far therefore as the experience
of this vovage extends, it appears that the errors of the log marked forty-five feet are al-
ways on the fafe fide, and that thofe of the longer marked line are always fhort of the
run; but that Bouguer’s is much more accurate than either.
It is not to be expected that the obfervations of a fingle voyage can be fufficient to
determine the merit of any inftrument, particularly one of fo much confequence as the
log. Ithought it right, however, to give an account of the trial I made of the different
methods, and of fuch remarks as occurred to me.*
I alfo tried two perpetual logs; one invented by Mr. Ruffell, the other by Foxon,
both conftructed upon this principle, that a fpiral, in proceeding its own length in the
direction of its axis through a refifting medium, makes one revolution round the axis ;
if therefore the revolutions of the fpiral are regiftered, the number of times it has gone
its own leneth through the water will be known. In both thefe the motion of the fpiral
in the water is communicated to the clock-work within board, by means of a fmall line,
fattened at one end to the {fpiral which tows it after the fhip, and at the other to a
fpindle which fets the clock-work in motion. ‘Vhat invented by Mr. Ruffell has a half
fpiral of two threads, made of copper, and a fmall dial with clock-work, to regifter the
number of turns of the fpiral. Foxon’s has a whole fpiral of wood with one thread,
and a larger piece of clock-work, with three dials, two of them to mark the diftance,
and the other divided into knots and fathoms, to fhew the rate by the half-minute glafs,
for the convenience of comparing it with the log. ;
This log, like all others, is liable to the firft error, as well as to the fecond. ‘The third
it partakes of in a very {mall degree, only affecting the reckoning by that quantity which
the {piral is thrown towards the {hip ; whereas in the log the fame circumftance affects
the whole rate for the hour. The fourth it is entirely free from, as well as the fifth.
It will have the advantage of every other in fmooth water and moderate weather, when
it is neceffary to ftand on one courfe for any particular diftance, efpecially in the night,
or a fog, as it meafures exactly the diftance run. It will alfo be very ufeful in finding
the trim of a fhip when alone; as well as in furveyinga coatt in a fingle fhip, or in mea-
furing diftances in a boat between headlands or thoals, when a bafe is not otherwife to
be obtained ; both which it will do with the greateft accuracy in fmooth water, witha
large wind, and no tide or current. But notwithftanding thefe advantages, which will
make it very ufeful and worth having, I doubt much whether it might ever be fubfti-
tuted entirely in the room of the common log. Machines eafily repaired or replaced
have advantages at fea, which fhould not lightly be given up for others more fpecious.
Obfervations on the Ufe of the Megameter in Marine Surveying.
Tue greateft difficulty in marine furveying is that of obtaining an accurate bafe, from
the extremities of which the angles may be taken with precifion, for afcertaining the
bearings and diftance of headlands and fhoals, when either want of time or other cirs
cum{tances make it impracticable to. land and meafure a bale. ‘The ufual way is, to
eftimate the diftance by the log, and to take the angles by the compafs. ‘This method
is liable to many errors, and affords no means of correcting or difcovering them. ‘The
megameter, con{tructed upon the principles of the object. glafs micrometer, defcribed by
M. de Charniere, and applied by him to find the longitude at fea, | thought might be
ufefully applied to marine furveying. ‘That which I ufed was made by Ramf{den, with
* The table and others are omitted, as only interefling to nautical readers.
7 fome
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