I
know only the rudiments of navigation. There is a vast deal yet for
me to learn. On the Snark there is a score of fascinating books on
navigation waiting for me. There is the danger-angle of Lecky,
there is the line of Sumner, which, when you know least of all where
you are, shows most conclusively where you are, and where you are
not. There are dozens and dozens of methods of finding one's
location on the deep, and one can work years before he masters it
all in all its fineness.
Even in the little we did learn there were slips that accounted for
the apparently antic behaviour of the Snark. On Thursday, May 16,
for instance, the trade wind failed us. During the twenty-four
hours that ended Friday at noon, by dead reckoning we had not sailed
twenty miles. Yet here are our positions, at noon, for the two
days, worked out from our observations:
Thursday 20 degrees 57 minutes 9 seconds N
152 degrees 40 minutes 30 seconds W
Friday 21 degrees 15 minutes 33 seconds N
154 degrees 12 minutes W
The difference between the two positions was something like eighty
miles. Yet we knew we had not travelled twenty miles. Now our
figuring was all right. We went over it several times. What was
wrong was the observations we had taken. To take a correct
observation requires practice and skill, and especially so on a
small craft like the Snark. The violently moving boat and the
closeness of the observer's eye to the surface of the water are to
blame. A big wave that lifts up a mile off is liable to steal the
horizon away.
But in our particular case there was another perturbing factor. The
sun, in its annual march north through the heavens, was increasing
its declination. On the 19th parallel of north latitude in the
middle of May the sun is nearly overhead. The angle of arc was
between eighty-eight and eighty-nine degrees. Had it been ninety
degrees it would have been straight overhead. It was on another day
that we learned a few things about taking the altitude of the almost
perpendicular sun. Roscoe started in drawing the sun down to the
eastern horizon, and he stayed by that point of the compass despite
the fact that the sun would pass the meridian to the south. I, on
the other hand, started in to draw the sun down to south-east and
strayed away to the south-west. You see, we were teaching
ourselves. As a result, at twenty-five minutes past twelve by the
ship's time, I called twelve o'clock by the sun. Now this signified
that we had changed our location on the face of the world by twenty-
five minutes, which was equal to something like six degrees of
longitude, or three hundred and fifty miles. This showed the Snark
had travelled fifteen knots per hour for twenty-four consecutive
hours - and we had never noticed it!
