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Updated: May 17, 2025
In the A.M. found the Variation, by the Amplitude, to be 11 degrees 25 minutes East, and by Azimuth 11 degrees 20 minutes. At Noon the observed Latitude was 38 degrees 51 minutes South, and Longitude made from Cape Farewell 16 degrees 45 minutes; Longitude in 202 degrees 43 minutes West; Course and distance sail'd since Yesterday noon South 76 degrees 45 minutes West, 96 Miles. Wednesday, 11th.
Knight's needles belonging to the Azimuth Compasses, all of which I judged to be good ones, and yet when applied to the Meridian line I found them not only differ one from another sometimes a degree and a half; but the same needle would differ from itself more or less, the difference sometimes amounting to half a degree, both at the same time and on differant days.
Thursday, 22nd. Genteel breezes and Clear weather. At 4 p.m. saw the Salvages bearing South; at 6, the Body of the Island bore South 1/2 West, distant about 5 leagues. Found the Variation of the Compass by an Azimuth to be 17 degrees 50 minutes West. At 10 the Isles of Salvages bore West by South 1/2 South, distance 2 leagues.
Hence we find him first making brass scales, parallel-rulers and quadrants. By the end of one month in this department he was able to finish a Hadley quadrant. From this he proceeded to azimuth compasses, brass sectors, theodolites, and other delicate instruments.
The same remark applies to the line joining the opposite blunt angles of a crystal of Iceland spar. The arrangement of the molecules round this line being symmetrical, the condition of the ether depending upon these molecules shares their symmetry; and there is, therefore, no reason why the wavelength should alter with the alteration of the azimuth round this line.
In this situation I found the variation to be 14 degrees 27' easterly, per azimuth.... I have several times passed within the Antarctic circle, on different meridians, and have uniformly found the temperature, both of the air and the water, to become more and more mild the farther I advanced beyond the sixty-fifth degree of south latitude, and that the variation decreases in the same proportion.
Our latitude was 38° 20' S., our longitude, by log, 147° 6' W., and the variation of the needle, by the azimuth, 7° 9' E. Among others that observed the comet, was Tupia, who instantly cried out, that as soon as it should be seen by the people of Bolabola, they would kill the inhabitants of Ulietea, who would with the utmost precipitation fly to the mountains.
First part light Breezes and Cloudy; Middle frequent heavy Squalls, with rain, till towards Noon when we had again little wind. Found the Variation by the mean of 3 Azimuth, taken this Morning, to be 8 degrees 52 minutes West, which makes the Variation found Yesterday doubtful.
He enlarged upon the science of navigation; treated us to a dissertation on Mercator's Sailing and the Azimuth compass; and went into an inexplicable explanation of the Lord only knows what plan of his for infallibly settling the longitude.
It is based on the fact discovered by Sir David Brewster, that the light of the sky is polarised in a plane at an angle of ninety degrees from the position of the sun. It follows that by discovering that plane of polarisation, and measuring its azimuth with respect to the north, the position of the sun, although beneath the horizon, could be determined, and the apparent solar time obtained.
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