"Very well, then," continued Smellie. "Be so good, Mr Mildmay, as to
draw a line south-south-east from that pencil-mark which represents the
_Daphne_ on your chart."
The master took his parallel ruler and did so.
"So far, so good," resumed the second lieutenant. "Now my sketch shows
that the outer extremity of Shark Point bore from the tree north-west ¼
west. In other words, the tree bears from Shark Point south-east ¼
east. Lay off that bearing, Mildmay, if you please."
"Very good," he continued, when this second line had been drawn. "Now
it is evident that the point where these two lines intersect must be the
position of the tree. But, as a check upon these two bearings I took a
third to that sharp projecting point at the mouth of Banana Creek,"
indicating with the pencil on the chart the point in question. "That
point bears north-west by north; consequently the tree bears from it
south-east by south. Mark that off also, Mildmay, if you please."
The master did so, and the three lines were found to intersect each
other at exactly the same point. "Capital!" exclaimed Smellie, in high
good-humour. "That satisfactorily establishes the exact position of the
tree. Now for the next step. The slave fleet bears north-west ¼ west
from the tree; and the western entrance to the creek (that by which we
shall advance to the attack to-night) bears exactly north-west from the
same point. Let us lay down these two bearings on the chart--thus. Now
it is evident that the slave fleet and the entrance to the creek are
situate _somewhere or other_ on these two lines; the question
is--_where_? I will show you how I ascertained those two very important
bits of information if you will step to my cabin and bring me the
telescope which you will find hanging against the bulkhead."
Intensely interested in this valuable practical lesson in surveying I
hurried away to do his bidding, and speedily returned with the glass, a
small but very powerful instrument, which I had often greatly admired.
Taking the telescope from my hand he drew it open and directed my
attention to a long series of neat little numbered lines scratched on
the polished brass tube.
"You see these scratches?" he said. "Very well; now I will explain to
you what they are. When I was a midshipman it was my good fortune to be
engaged for a time on certain surveying work, during which I acquired a
tolerably clear insight of the science. And after the work was over and
done with, it occurred to me that my knowledge might be of the greatest
use in cases similar to the present. Now I may tell you, by way of
explanation, that surveying consists, broadly, in the measurement of
angles and lines. The angles are, as you have already seen, very easily
taken by means of a pocket-compass; but the measurement of the lines
bothered me very considerably for a long time. Of course you can
measure a line with perfect accuracy by means of a surveyor's chain, but
I wanted something which, if not quite so accurate as that, would be
sufficiently correct, while not occupying more than a few seconds in the
operation of measurement. So I set to work and trained myself to judge
distances by the eye alone; and by constant diligent practice I acquired
quite a surprising amount of proficiency. And let me say here, I would
very strongly recommend you and every young officer to practise the same
thing; you will be surprised when you discover in how many unexpected
ways it will be found useful. Well, I managed to do a great deal of
serviceable work even in this rough-and-ready way; but after a time I
grew dissatisfied with it--I wanted some means of measuring which should
be just as rapid but a great deal more accurate. I thought the matter
over for a long time, and at last hit upon the idea of turning the
telescope to account. The way I did it was this. You have, of course,
found that if you look through your telescope at an object, say, half a
mile away, and then direct the instrument to another object, say, four
miles off, you have to alter the focus of the glass before you can see
the second object distinctly. It was this peculiarity which I pressed
into my service as a means of measuring distances. My first step was to
secure a small, handy, but first-rate telescope--the best I could
procure for money; and, provided with this, I commenced operations by
looking through it at objects, the exact distances of which from me I
knew. I focused the glass upon them carefully, and then made a little
scratch on the tube showing how far it had been necessary to draw it out
in order to see the object distinctly; and then I marked the scratch
with the distance of the object. You see," pointing to the tube, "I
have a regular scale of distances here, from one hundred yards up to ten
miles; and these scratches, let me tell you, represent the expenditure
of a vast amount of time and labour. But they are worth it all. For
instance, I want to ascertain the distance of an object. I direct the
telescope toward it, focus the instrument carefully, and find that I can
see it most clearly when the tube is drawn out to, say, this distance,"
suiting the action to the word. "I then look at the scale scratched on
the tube, and find that it reads six thousand one hundred feet--which is
a few feet over one nautical mile. And thus I measure all my distances,
and am so enabled to make a really satisfactory little survey in a few
minutes as in the case of this afternoon. You must not suppose,
however, that I am able to measure in this way with absolute accuracy; I
am not; but I manage to get a very near approximation to it, near enough
for such purposes as the present. Thus, within the distance of a
quarter of a mile I have found that I can always measure within two feet
of the actual distance; beyond that and up to half a mile I can measure
within four feet of the actual distance; and so on up to ten miles,
which distance I can measure to within four hundred feet.
"And now to return to the business in hand. My telescope informed me
that the slave fleet was anchored at a distance of eighteen thousand
three hundred feet (or a shade over three nautical miles) from the tree,
and that the western entrance to the creek is twenty-eight thousand nine
hundred feet (or about four and three-quarter nautical miles) from the
same spot. We have now only to mark off these two distances on the two
compass-bearings which we last laid down on the chart: thus,"--measuring
and marking off the distances as he spoke--"and here we have the
position of the slavers and of the entrance to the creek; and by a
moment's use of Mildmay's parallel ruler--thus--we get the compass-
bearing of the entrance from the _Daphne_. There it is--south-east by
east; and now we measure the distance from one to the other, and find it
to be--eight miles, as nearly as it is possible to measure it. Thus,
you see, my rough-and ready survey of this afternoon affords us the
means of ascertaining our course and distance from the _Daphne_ to a
point for which we should otherwise have been obliged to search, and
which we could not possibly have hoped to find in the impenetrable fog
which now overspreads the river."
"Thank you, Mr Smellie," said I, highly delighted with the lesson I had
received; "if it will not be troubling you too much I think I must ask
you to give me a lesson or two in surveying when you can spare the
time."
"I shall be very pleased," was the reply. "Never hesitate to come to me
for any information or instruction which you think I may be able to
afford you. I shall always be happy to help you on in your studies to
the utmost extent of my ability. But we have not quite finished yet,
and it is now, Mildmay, that I think _you_ may perhaps be able to help
us. You see we shall have to pull--or sail, as the case may
be--_across_ the current, and it will therefore be necessary to make
some allowance for its set. Now do you happen to know anything about
the speed of the current in the river?"
"Not half so much as I should like," replied the master; "but a hint
which the skipper dropped this morning caused me to take the dinghy and
go away out in mid-stream _to spend the day in fishing_--ha--ha--ha!
The Yankee had his glass turned full upon me, off and on, the whole
morning--so I'm told--and if so I daresay he saw that I had some fairly
good sport. But I wasn't so busy with my hooks and lines but that I
found time to ascertain that the ebb-stream runs at a rate of about four
knots at half-tide; and just abreast of us it flows to seaward at the
rate of about one knot at half-flood; the salt water flowing _into_ the
river along the bottom, and the fresh water continuing to flow
_outwards_ on the surface. Now, at what time do you propose to start?"
"About half-past nine to-night," answered Smellie.
Old Mildmay referred to a book by his side, and then said:
"Ah, then you will have about two hours' ebb to contend with--the last
two hours of the ebb-tide. Now let me see,"--and he produced a sheet of
paper on which were some calculations, evidently the result of his
observations whilst "Sshing." He ran over these carefully, and then
said:
"How long do you expect it will take you to cross?"
"Two hours, if we have to pull across--as I expect we shall," answered
the second lieutenant.
"Two hours!" mused the master. "Two hours! Then you'll have to make
allowance, sir, for an average set to seaward of two miles an hour all
the way across, or four miles in all."
"Very well," said Smellie. "Then to counteract that we must shape our
course for a point four miles _above_ that which marks the entrance to
the creek--must we not, Mr Hawkesley?"
"Certainly," I said; "that is quite clear."
"Then be so good as to lay that course down on the chart."
I measured off a distance of four miles with the dividers, and marked it
off _above_ the mouth of the creek; then applied the parallel ruler and
found the course.
"It is exactly south-east," said I; "and it will take us close past the
southern extremity of this small island."
"That is quite right," remarked Smellie, who had been watching me; "and
if we happen to sight the land in passing that point it will be an
assurance that, so far, we have been steering our proper course. But--
bless me,"--looking at his watch--"it is a quarter after nine. I had no
idea it was so late. Run away, Mr Hawkesley, and make your
preparations. Put on your worst suit of clothes, and throw your pea-
jacket into the boat. You may be glad to have it when we get into the
thick of that damp fog. Bring your pistols, but not your dirk; a ship's
cutlass, with which the armourer will supply you, will be much more
serviceable for the work we have in hand to-night."
I hastened away, and reached the deck again just in time to see the men
going down the side into the boats after undergoing inspection.