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Updated: August 17, 2024
These things, I confess, did puzzle me: neither was I fully satisfied as to the exactness of the taking the variation at sea: for in a great sea, which we often meet with, the compass will traverse with the motion of the ship; besides the ship may and will deviate somewhat in steering, even by the best helmsmen: and then when you come to take an azimuth there is often some difference between him that looks at the compass and the man that takes the altitude height of the sun; and a small error in each, if the error of both should be one way, will make it wide of any great exactness.
Our dead reckoning had heretofore always given the same results as our astronomical observations. During the last eight days of our march we had continuous sunshine. Every day we stopped at noon in order to measure the meridian altitude and every evening we made an observation for azimuth.
Wednesday, 30th. Fresh breeze and fair weather. At 1 a.m. saw the Comet a little above the Horizon in the East. It pass'd the Meridian about 1/2 past 4; the Tail of the Comet Subtended an Angle of 42 degrees. At 8 a.m. Variation per Azimuth 7 degrees 9 minutes East. Bent another suit of Sails.
Now, it was plainly a labor of love for Captain Sleet to describe, as he does, all the little detailed conveniences of his crow's-nest; but though he so enlarges upon many of these, and though he treats us to a very scientific account of his experiments in this crow's-nest, with a small compass he kept there for the purpose of counteracting the errors resulting from what is called the "local attraction" of all binnacle magnets; an error ascribable to the horizontal vicinity of the iron in the ship's planks, and in the Glacier's case, perhaps, to there having been so many broken-down blacksmiths among her crew; I say, that though the Captain is very discreet and scientific here, yet, for all his learned "binnacle deviations," "azimuth compass observations," and "approximate errors," he knows very well, Captain Sleet, that he was not so much immersed in those profound magnetic meditations, as to fail being attracted occasionally towards that well replenished little case-bottle, so nicely tucked in on one side of his crow's nest, within easy reach of his hand.
Packed away in his wallet lay geological hammer, azimuth compass, clinometer, miniature microscope, why should he drag all that lumber about with him? What to him were the bygone millions of ages, the hoary records of unimaginable time? One touch of a girl's hand, one syllable of musical speech, was it not that whereof his life had truly need?
Found the Variation by a great Number of Azimuth made this Afternoon to be 8 degrees 21 minutes 30 seconds West, and by the Morning Amplitude 7 degrees 48 minutes. At Noon try'd the Current, and found it set North-North-West 3/4 West, 1 1/8 miles per hour.
The first part light Airs and Calm; Middle, moderate breezes and Cloudy; latter part very squally with rain. A.M. Variation per Azimuth 7 degrees 18 minutes East. At Noon took in the Topsails and got down Topgallant yards.
Variation per Azimuth 16 degrees 12 minutes, per Amplitude 16 degrees 32 minutes; Mean of the Two 16 degrees 22 minutes East. Between 9 and 10 a.m. took 7 sets of Observations between the sun and moon to find the Longitude of the Ship. Each set Consists of three Observations; the Mean of the whole gave 59 degrees 18 minutes 34 seconds West of Greenwich.
Kennedy was talking rapidly of data which he had derived from his metric photograph, from plumb line, level, compass, and tape, astronomical triangle, vertices, zenith, pole and sun, declination, azimuth, solar time, parallactic angles, refraction, and a dozen bewildering terms. "In spherical trigonometry," he concluded, "to solve the problem three elements must be known. I knew four.
The variation of the compass I found to be 4° 46' easterly, this being the result of a great number of trials made with four of Dr Knight's needles, adapted to azimuth compasses.
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