CHAPTER XVII.
Make an astrolabe, and obtain the latitude of the island, and, by an eclipse of the moon, the longitude also. By means of the Epitome make a chart on Mercator's projection, and find out the distance from any known land.
I found in my book a description of an instrument used by the ancients to ascertain altitudes and to measure angles, called an Astrolabe, which, upon careful study and examination of the cut, I felt confident would serve my purpose admirably. So to work I went, and in this manner. I made first a strong four-legged stool or bench, about three feet in height and four feet long, and two feet wide upon the top. I then took some nice planed pieces of my dark hardwood and made a smooth surface about an inch thick and five feet square. On this, afterwards to be erected on the stool at right angles like an inverted letter T, I drew a circle with a pair of immense dividers that I made for this purpose, taking in all the area possible, which made my circle about fifty-nine inches in diameter, leaving a margin of one inch,—supposing my inches to have been of the right length; and this I determined by the length of the knuckle of my thumb, which I formerly used for quick measurement, and from which standard I constructed the only rule I ever have used on the island. How nearly correct it is I have no means of knowing. This groove I impressed into the wood by repeatedly turning the dividers around the circumference. I then went to work and subdivided this circle into degrees and minutes, which I did by marking the circle once across at any angle passing through the centre mark, and then by another mark crossing this one at exactly right angles, which I determined by means of my dividers—as laid down in Bowditch's Epitome—by the use of them at equal distances from the centre on the line already marked, sweeping them till the two lines crossed beyond the circumference, making a small mark there so as to erect a perpendicular on the base already drawn. This cut my circle at once into quadrants of 90° each, and these were subdivided again in like manner. I made the circle large on purpose, so as to be able to mark it plainly to sections of one minute each, and by its size to avoid any error in any angle, the chances of which were greatly decreased by every inch of diameter. As I constructed it, I had nearly one-half an inch of circumference to mark sixty minutes upon, and as I only subdivided one of the quadrants it did not take me very long, each degree being represented by a space slightly smaller than a half inch, which was a good large scale to work upon. Having finished the marking of my board I nailed it firmly to the stool in an upright position, with the quadrant, that I had carefully subdivided on the marked circumference, pointing with one of its angles to zenith, and the other on a line with the top of the stool. I then procured a nice straight piece of cane some six feet in length and about an inch in diameter, and with a heated rod of iron burned out all the pith between each joint till I had made a nice tube of that length. Just within the aperture at one end I fastened with a little fish glue a large strong hair from the beard of one of my goats. I then fastened, by means of a hole through the centre of my upright disc, this tube or telescope to it on the side that was subdivided into degrees, and about an inch from the face. I fastened this so that it was held firmly in place, and yet could be moved upon its centre by the pressure of my hand on either arm. This tube I then furnished with a small delicate pin on the outside, in an exact line with the stretched hair inside the tube, and pointing to the degrees and minutes on the marked circumference on the disc, which it almost touched. In other words, if I moved one arm of the tube, the needle on the outside would follow the grooved circumference on the disc, and upon being released would mark some given degree or minute. Having gotten this machine all in order and complete, I placed it one day so as to examine the sun near noon, and here is how I obtained my latitude. What I was doing now was not so very difficult. I well knew that there were several ways to determine latitude. I was aware that the difference of a minute or two even in my altitude, as apparently observed, would not disturb my computation more than a mile or so. In fact each minute marked upon the disc practically stood for one mile of latitude, and the mean of several observations would correct even any errors from this cause.
I waited till I knew that it was nearly noon by the appearance of the sun, and then commenced operations. In the first place I aimed my tube at the sun, and to be able to do so without injuring my eye, I would say that I had fitted the orifice of the tube nearest to me with a piece of smoke-colored membrane or backbone of the squid, which is as transparent as glass, and very thin and delicate. Having, by moving the tube with my hand, brought the sun so that it seemed to stand upon the hair in the outer end of the tube, or like a great capital O upon a base line O, I left it carefully in that position for a moment or two, and then applied my eye again, and found, as I supposed, that the sun no longer seemingly rested upon the hair in my tube, but had risen, which forced me to again lower the arm nearest me and elevate the other extremity, and proved to me that it was not yet noon, and that the sun had not yet reached the meridian. This I did many times, till at last the sun seemed for a minute or so to stand still, as sailors say, and I knew that it was at meridian. I took good care not to touch the instrument, but waited quietly till, by glancing through it, I saw in a few moments the disc of the sun, or lower limb as it is termed, begin to drop below the hair in my tube, and I was then positive that it had passed the meridian. Being assured of this, I went carefully to the marked circumference on the upright disc and noted carefully the degree and minute to which the needle in the side of the tube pointed, which in this case was 54° 51′. Having carefully marked this down with ink upon birch bark, I went again to the other end of the tube, and, elevating it, brought the outer end down toward the sea till the hair and the horizon seemed to be one. I then again carefully observed the degree and minute at which the needle pointed, which in this case was 7° 16′, and my task was done; for, by subtracting 7° 16′ from 54° 51′ I obtained 47° 35′, which was exactly the apparent altitude of the sun at noon on September 22, 1866, civil account; and, having that, it was easy to determine the latitude in the following manner:—
| At noon observe the altitude of the sun's lower limb bearing North | 47° 35′ |
| Add for semi-diameter, dip, etc. | 12′ |
| 47° 47′ | |
| Subtract from | 90° 00′ |
| Sun's zenith distance | 42° 13′ S. |
| Declination for longitude, say 115° W | 8′ S. |
| Latitude by observation | 42° 21′ S. |
Thus I demonstrated the latitude of my island; but now for the longitude. To obtain that I knew that I must first ascertain the time at the island: I could do nothing without that; for longitude was, as I well knew, simply time changed into degrees. I thought of fifty different ways to obtain correct time, but believed none of them sufficiently accurate for my purpose. I could make a sundial for one thing, find out the length of the day by the Epitome and Nautical Almanac, make candles to burn such a length of time, sand to run down an inclined plane at such a rate, but none of these would do.
The difference of a minute, or one-sixtieth of a degree, in an observed altitude would only affect, as I have said, my latitude just one mile, whilst an error of time of one minute from true time would, as I was well aware, throw my longitude out just fifteen miles; hence it behoved me to have exact time if I desired to get exact longitude, and therefore I saw nothing for it but that I must construct a clock, and at it I went. It was not such an enormous undertaking after all. Of course I should make it of wood, and in my boyhood I knew many wooden clocks that kept excellent time; besides, if I could only construct something that would keep time for an hour or two without much error, it would answer my purpose. If I had a clock that I could set at noon by my observation, nearly correct, I could correct it perfectly by an afternoon observation, and have for an hour or so true time, even if it did gain or lose a few minutes in twenty-four hours. So to work I set, and soon turned out the few small wheels necessary, and the weight and pendulum for the same. I spent little time upon the non-essentials, but put it together inside my house on the wall, open so that I could get at it, and furnished it with wooden hands and a thin wooden face.
After I had arranged it and found that it would tick, and by observations at noon for a few days been able to regulate the pendulum, I went diving into the Epitome and the Nautical Almanac as to how I should utilize it so as to get my longitude, after all; when one evening, in turning over the Nautical Almanac, which was calculated for 1866, 1867, 1868, and 1869, my eye fell upon the following, and I felt that my task was done:—
Total eclipse of the moon, September 30,1866, invisible at Greenwich, visible in South America, South Pacific Ocean, and parts of Africa, Asia, and Indian Ocean.