Letter to the Editor


Quebec Mercury #41, Page 241. Tuesday, May 22, 1827.

  Mr. Editor:  Several of the ships coming up the Gulf and River
this spring, after having had a good departure from the Bird
Isles, suddenly made the land about Cape Rosier, and as far south
as Malbaie (to the southward of Gaspé), when they expected they
were in a fair way between the Island of Anticosti and the south
land.  This deserves the serious attention of all nautical men
using this trade.  And it is also their duty, if possible, to
point out the cause of such error in their courses steered.
  The causes appear to me to be, first; the strong southeast
current which runs down this river in the spring of the year, at
the rate of 3 or 3Ť knots; which current, as soon as it rounds
Cape Rosier and Gaspé, sets directly to the southward with equal
force.
  Secondly, it would be well that all ships at every opportunity
should try experiments on the variation of the compass.  I am
fully of opinion that it does, and has increased.  Since my first
coming up the St. Lawrence, and very lately from experiments
made, I found six degrees more variation than ever I expected, of
my courses steered.
  Thirdly, the remarks and experiments of the late truly
scientific officer, Captain Matthew Flinders, of the Royal Navy. 
Those who may be in possession of his work on this subject, will
excuse my transcribing them, for those who have them not.  His
experiments seems to me to be simple, and many a master, I think,
would amuse himself with them, if in possession of his rules, &c. 
He clearly convinces every nautical man that the ship itself has
a certain local attraction on the compasses, and that, that
attraction is always greatest, when the ship's head is near or at
the east and west points of the compass.
  He says, "When the head was east, the variation differed from
the true, and always on the same side, while the ship remained in
the same hemisphere.  2nd; When the head was west, the difference
was equally great, but a contrary way.  3rd;  The head being
north or south made no difference in the variation, and it was
then a meridian between what was found at east and west.   4th; 
At the intermediate points between the magnetic meridian and east
or west, the difference from the true variation bears a
proportion to the angle made by the ship's head with the
meridian.  If the head were on the western side, the difference
was of the same nature as that when the head was west; if on the
east side, as at east.  5th;  The proportion at the intermediate
points, obeyed the following law.
  As the radius is to the difference at east or west (for 8
points), so is the sine of the angle between the ship's head and
magnetic meridian, to the difference for that angle.
  Or if the number of points which the head was to the right or
left of the meridian were taken as a course, and difference for
eight point reduced to minutes, taken as the distance; then the
difference for the number of point was found in the departure
column of the traverse tables."
  And he says, "that these differences were of a contrary nature
in the southern, to what they were in the northern hemisphere". 
And that "the causes of all these changes in a compass on
shipboard, and the modes by which they may be obviated, I have
endeavoured to explain, in the account of the experiments drawn
up, by order of the Admiralty.  And to obtain further proofs, it
is desirable that the differences should be observed in as many
ships and as accurately as possible, in the following manner:
  The azimuth compass to be used should be a good one.  The card
traversing freely, and, if possible, the needle should be
retouched, with magnetic bars, before the observation is made.  A
low stand, or stool must be prepared, so that when the compass is
placed upon it, it may be of the same height with that by which
the ship is steered.  The binnacle being then taken away,
substitute the stand and azimuth compass; or if the sun cannot be
then seen all round, fix the stand as near to the situation as it
can be seen, let it be clear of any iron, and exactly amidships. 
Let the azimuth be taken, using both sides of the vane, and this
as often as convenient, and with the ship's head in various
directions; but more particularly, at as near east or west,
noting the direction of the head to the nearest quarter point.
  These observations should always be made with the same
instrument and in the same place, and be entered in a table of
ten columns under the following heads:
  1. Time.
  2. Latitude.
  3. Longitude.
  4. Dip of the needle, if an instrument is on board.
  5. Altitude of sun's centre, corrected for dip, refraction, and
semidiameter.
  6. Azimuth observed, being the mean of three or four sights,
with each side of the vane.
  7. By whom made.
  8. Ship's head, mean of what it was at the beginning and end.
  9. Variation.
  10. Circumstances specifying, if at anchor or under sail, also
if the ship was steady or at anchor.
  A number of such experiments carefully made, and particularly
when lying steadily at anchor, would show whether the difference
at the binnacle is usually so great as to cause much error, and
consequently, how far the discovery of them is of importance to
ordinary navigation.  But more specially to the navigation of the
Gulf and River St. Lawrence.
  As many of the masters in this trade will have time, and
perhaps take pleasure in making the above experiments, by their
joint abilities, they would correctly ascertain the true
variation to be allowed both coming up and going down this river,
and in time prevent any such error in the compass as that now
complained of.
  Taking all these causes into consideration, I am of opinion
that a local attraction added to the increase of variation, have
been the cause that so many ships found themselves so much
farther to the southward than they thought themselves, and it
appears in many instance, very fortunate that daylight appeared
so soon or they might have incurred much serious danger.  The
chief courses steered seem to have been north northwest by west,
and north northwest.  Which courses everyone would think fair
courses, according to the distance from the Bird Rock, but it
proved erroneous.  Therefore, if a ship in future should not be
certain of her distance from the Birds, she may safely, in my
opinion, steer northwest Ť north, and northwest, particularly in
the spring and fall of the year, but the courses down from Cape
Rosier may be used as usual; say one point of the compass
difference from the opposite point steered upwards.
                                             A Marine.
  Quebec, May 21, 1827.