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564 PHIPPS’S JOURNAL.
feet with a ¢lafs of thirty feconds; but he rather recommends reftoring the half minute
glafs, and making the correétion on the line. Since that time the feamen, whether from
finding the allowance of one foot in fifty not a fufficient compenfation for the accidental
errors to which the log is fubje&t, or from a preference of a meafure nearly equal to
the ftatute mile, have ufed a line of forty-five feet to thirty feconds, or a glafs of
twenty-eight feconds to forty-two feet.
All the writers J have met with, who have treated of the log, except Wilfon, have
complained of the feamen not having adhered to Norwood’s meafure. Norwood him-
felf, however, feems to have been aware of the neceflity of fubmitting to the teft of
experiment the advantages of a new meafurement derived from theory. In the preface
to his Seaman’s Practice he fays, “* Becaufe I am perfuaded we have at this day as
many excellent navigators in this kingdom, and as great voyages performed, as from
any other place in the world, I fhould be glad to hear of the experimental refolution of
this problem by fome of them, though it were but running eight or ten degrees near the
meridian ; for fo I doubt not but what I have here written thereof would receive fur-
ther confirmation and better entertainment than "happily it will now, being fo much
different from the common opinion.”
Had the errors in the diftance arifen only from a fault in marking the line, nothing
would have been more eafy than to have removed that difficulty by comparing care-
fully the different meafures with the obfervations, and adhering to that which had
been found to correfpond beft with them. But the diftance meafured by the log
being rendered uncertain by many accidental circumf{tances, it becomes difficult, or
rather impoffible, to find any length of line which will fhew invariably the diftance run
by the fhip, or even to afcertain with precifion that meafure which will at all times
come neareft the truth. Some of thefe circumftances are :
1. The effcéts of currents.
2. The yawing of the thip going with the wind aft, or upon the quarter, when fheis
feldom fteered within a point each way: this | mention as an error in the diftance, and
not in the courfe ; fince, though the fhip by being yawed equally each way may make
the intended courfe good upon the whole, yet the diftance will be fhortened as the verfed
fine of the angle between the line intended and that fteered upon.
3. By the fhip being driven on by the f{well, or the log during the time of heaving
being thrown up nearer the fhip.
4. By the log coming home, or being drawn after the fhip, by the friction of the
reel and the lightne!s of the log. Norwood mentions thefe two lalt, and fays, ‘¢ For
thefe caufes, it is like, there may fometimes be allowed three fathoms or more than is
veered out; but this, (as a thing mutable and uncertain) being fometimes more, fome-
times lefs, cannot be brought to any certain rule, but fuch allowance may be made asa
man in his experience and difcretion finds fit.”’
5. By the log being only a mean taken every hour, and confequenty liable to error
from the variations in the force of the wind during the intervals, for which an arbitrary
correction is made by the officer of the watch; and though men of {kill and experience
come near the truth, yet this allowace muft, from its nature, be inaccurate.
Thefe circumftances did not efcape M. Bouguer’s attention, and his ingenuity fug-
gefted to him an improvement of the common log, which would correc the errors
likely to arife from the moft material of thefe circumftanees : a defcription of this im-
provement he publifhed at large in the Memoirs of the Academy of Sciences for the
year 1747; it has fince been abridged in the edition of his Navigation by De la Gaille.
Jt appears extraordinary that this log fhould never have been made ufe ot by others -
8 the
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