[Illustration: Wollaston's Dip Sector]
In our tables for apparent dip of the visible horizon at different heights from the sea, as calculated from the known curvature of the earth, allowance is made for the refraction of the atmosphere, on a supposition of its being constant, but as it is known to vary, the tabular dip will often be erroneous, and, consequently, altitudes taken under different states of the atmosphere, will exhibit different instead of corresponding results.
It is foreign to the present purpose to shew what the causes are which have most effect in raising or depressing the apparent horizon. It may be sufficient to mention, that changes in the relative temperature of the air and the sea must produce changes in the refraction near the surface. Dr. Wollaston has published two papers in the Philosophical Transactions on this subject, in the volumes for 1800 and 1803, and to these I beg to refer the reader for precise information upon this very curious subject.
The object which this sector proposes to attain, is the actual admeasurement of the dip angle; that is, to ascertain how much the visible horizon is depressed below the horizontal plane passing through the eye of the observer. The instrument is so contrived as to measure double the dip angle twice over, so that we obtain four times the required dip, and one quarter of this angle is what must be applied to vertical angles, measured from that part of the horizon which has been observed.
Figure I. is the instrument seen in perspective, and Fig. II. is a plan of it with the telescope removed. In order to explain its use, let A and B (Fig. II.) represent the two reflecting glasses at right angles to the plane of the instrument, and also nearly at right angles to each other. It is clear that when the plane of the instrument is held vertically, an eye situated at E, and looking through the unsilvered part of the glass A at a distant point C, will at the same time see by joint reflection from both glasses, another distant point D at 180º from C; and D will appear to correspond with C, if a suitable motion be given to the index glass B by the tangent screw F.
The instrument may now be supposed to measure the arc CZD. If the points C and D be each three minutes farther from the zenith than 90º, the entire angle will then exceed 180º by double that quantity. The relative position of the glasses then corresponds to 180º 6', and the six minutes of excess would be shewn on the arc at F if there were no index error. But, by reason of the index error, the real quantity will not be known till a similar observation has been made with the instrument in an opposite direction.
If the instrument be now inverted, so that the unsilvered glass is uppermost, the arc intended to be measured is CND, or the sum of the distances of the points C and D from the Nadir instead of the Zenith, which of course falls short of 180º by as much as the former arc exceeded that quantity.
The difference of the two arcs is consequently twelve minutes, and if the index be now moved till the objects C and D appear to correspond, the amount of this double difference will be shewn by thechange of positionof the vernier.
Hence it is evidently unnecessary that the index error should be previously known, and even preferable that its amount should be such as to avoid the needless introduction of negative quantities by positions on different sides of zero.
In the preceding description, it is supposed that the eye is looking directly through the unsilvered glass at the horizon, and that it also perceives the opposite horizon after two reflections; but an inspection of the figure will shew that the observer's head would necessarily intercept the rays from the horizon behind him. To obviate this, both the direct and the reflected rays are received in coming from the unsilvered glass, (and after passing through the field-glass of the telescope) on a mirror placed at an angle of 45º, which reflects them to the eye. By this ingenious contrivance, the obstruction is removed, and the opposite points of the horizon may be both seen at one moment.
In practice, it is most convenient to direct the telescope to the same part of the horizon in both cases. Thus, if the east and west parts of the horizon be observed, and that the index glass be uppermost, and telescope pointing to the west, the observer is on the south side, and his face must be turned to the north. When the instrument is inverted, if the observer turn himself round at the same time, so as to face the south, then the telescope will be pointed as before to the west; but since the index glass is now undermost, the inferior arc will now be measured precisely as if his face were to the north, but with the advantage of the same lights seen in the erect position of the instrument.
In using this instrument at sea for the first time, considerable difficulty arises from the constant change in the plane of the instrument from the perpendicular position, in which it is absolutely necessary that it should be held, in order to obtain a correct observation. What at first appears to be a defect, however, is a real advantage, namely, that whenever it is held in the least degree out of the vertical plane, the two horizons (that seen direct, and the reflected one) cross each other, and it is only when the plane is vertical that the horizons can appear parallel.
The object is to get the two horizons to coincide exactly, and for this purpose it will often be necessary to have them of different shades. This is managed, as in the sextant, by means of the screw, which raises or lowers the telescope. When the telescope is brought nearer to the plane of the instrument, the reflected horizon becomes dark and distinct, but when screwed off it becomes fainter, and is not so well defined. Practice alone can teach the degree of intensity which is most favourable. In general it is best to have one horizon dark, and the other light; then bring them very nearly to coincide, and wait till the ship is steady, at which moment a slight touch of the tangent screw brings them exactly to cover one another. It will happen, of course, that when the coincidence is perfect, there is only one horizon to be seen, and a doubt remains whether all is right, but a slight motion of the instrument, by making the horizons cross each other, defines them at once.
It is advisable to take several observations, and the safest way is to take one first with the index glass uppermost, and then with the instrument inverted, after which to return to the first, and so on for two or three times each way.
In the pages which follow, there is given a table containing the result of all the observations made during this voyage, preceded by several sets of observations in the fullest detail. From the table it will be observed how seldom the dip, actually measured, agrees with that inferred from the mean refraction. Some of these experiments shew very remarkable differences, and point out the great utility of this instrument.
The practical navigator, particularly if he has been in hot climates, will recollect how discordant his observations for latitude always were, and how few even of the best observers agree in their determination of the latitude of the same place, simple as the observation is thought to be. The cause is quite clear; and though it equally affects altitudes taken for absolute time, the disagreement is less obvious, and it will often happen that a chronometer going extremely well appears to vary every day from inaccuracy in the observations. Thus it is, I think, generally admitted, that it is almost impossible to rate a chronometer from altitudes observed with the sea horizon. Nor is this difficulty removed by taking equal altitudes, because the refraction in all probability will be different at the two observations. With an artificial horizon, indeed, the changes in refraction are not felt, because, at a considerable elevation above the horizon, the changes are very trifling. But it often happens in practice, that the artificial horizon cannot be used, and we are then reduced to the sea horizon, where the changes of refraction are always the greatest. In the Yellow Sea, for instance, we had no opportunity of landing during all the time that the squadron was at anchor, till the day before we sailed. So that during nearly a fortnight that the ships were at anchor, the sea horizon was necessarily used. I need only to refer to the observations taken off the Pei-ho, viz. from No. 37 to 62, to shew how extremely fallacious the results must have been.
It is much to be wished that this excellent instrument should be brought into general use in navigation.
No. 31.
July 23, 1816.—6 P.M.
Index uppermost. Instrument inverted.
A + 8'. 10" B - 7'. 10"8 . 05 7 . 108 . 00 7 . 10——— ———Mean 8 . 05 Mean 7 . 10 B.Mean + 8 . 05 A.———-15 . 15———-3 . 49 Dip.3 . 50 Tabular.1 Difference.———-
Height of the eye, 15 feet, 3 inches.
Parts of the horizon observed, WSW. and ENE.
Barometer 29 . 78 inchesThermometer {Air 82º{Sea 77ºLatitude 35º north.Longitude 124º east.
Wind light from south; horizon uncommonly well defined and sharp; sky clear, and sea perfectly smooth.
No. 40.
July 29, 1816.—9 A.M.
Index uppermost. Instrument inverted.
A + 8'. 20" B - 11'. 40"8 . 45 11 . 358 . 30 11 . 50——— ———-Mean 8 . 32 Mean 11 . 42 B.Mean + 8 . 32 A.———-4) 20 . 145 . 3 Dip.3 . 50 Tabular.1 . 13 Difference +———-
Height of the eye, 15 feet, 3 inches.Parts of the horizon observed, NW. and SE.The low land just visible in the NW. distant 12 or 14 miles.
Depth of the sea, 18 feet.Barometer 29 . 60 inches.Thermometer {Air 81º{Sea 84ºLatitude 38º. 50' north.Longitude 118º. 00' east.
There has been little wind this morning, after a very close night.
No. 43.
August 6, 1816.—1 P.M.
Index uppermost. Instrument inverted.
A + 7'. 48" B - 11'. 55"7 . 48 11 . 457 . 55 11 . 45——— ———-Mean 7 . 50 Mean 11 . 48 B.Mean + 7 . 50 A.———-4) 19 . 38———-4 . 54 Dip.3 . 53 Tabular.1 . 1 Difference +———-
Height of the eye, 15 feet, 6 inches.Parts of the horizon observed, SW. by S. and NE. by N.Depth of the sea, 29 feet.
Barometer - 29 . 64 inches.Thermometer { Air 83-1/2º{ Sea 81-1/2ºLatitude - 38º 50' north.Longitude - 118º 00' east.
Moderate breeze from SE. by S.; rather hazy, but the horizon sharp and distinct.
No. 50.
August 8, 1816.—6.15. A.M.
Index uppermost. Instrument inverted.A + 10'. 20" B - 12'. 50"10 . 18 12 . 4510 . 35 13 . 00————— —————Mean 10 . 24, 3 Mean 12 . 51.7 B.Mean + 10 . 24.3 A.—————4) 23 . 165 . 49 Dip.3 . 50 Tabular.1 . 59 Difference +—————
Height of the eye, 15 feet, 3 inches.Parts of the horizon observed, NNE. and SSW. clear of the land.Depth of the sea, 26 feet.
Barometer 29 . 65 inches.Thermometer { Air 69-1/2º{ Sea 78º
Wind NNW. moderate.Latitude - 38º. 50' north.Longitude - 118º. 00' east.
Mem.—The top of the fort at the mouth of the river, is just visible at 30 feet from the surface of the water.
No. 53.
August 10, 1816.—6.10. A.M.
Index uppermost. Instrument invertedA + 13'. 55" B - 7'. 30"13 . 50 7 . 1513 . 45 7 . 25———- ————-Mean 13 . 50 Mean 7 . 23.3 B.————-Mean + 13 . 50 A.————-4) 21 . 13.35 . 18.3 Dip.2 . 20 Tabular.2 . 58 Difference +
Height of the eye, 5 feet, 6 inches.Depth of the water, 5 feet.Parts of the horizon observed, N. by E. and S. by W. just clear of the land.
Barometer 29 . 69 inches. } On board His Majesty's ship Lyra,Thermometer { Air 75º } distant three or four miles.{ Sea 77º }Wind WNW.
About one mile from the fort of Tung-coo, at the entrance of the Pei-ho river.Note.—Instrument readjusted.
No. 58.
August 10, 1816.—2 P.M.
Index uppermost. Instrument inverted.
A + 15'. 40" B - 8'. 50"15 . 30 8 . 5015 . 35 8 . 50———— ————Mean 15 . 35 Mean 8 . 50 B.Mean + 15 . 35 A.
4) 24 . 25 6 . 6.3 Dip. 3 . 50 Tabular. 2 . 16 Difference +
Height of the eye, 15 feet, 3 inches.Parts of the horizon observed, N. by E. and S. by W.Depth of the sea, 27 feet.
Barometer 29 . 68 inches.Thermometer } Air 84º} Sea 83º
Latitude 38º . 50' north.Longitude 118º . 00' east.
No. 59.
August 10, 1816.—2. 15. P.M.
Index uppermost. Instrument invertedA + 15'. 10" B - 7'. 50"15 . 10 8 . 1015 . 10 8 . 05————- ————-Mean 15 . 13.3 Mean 8 . 01.7 B.Mean + 15 . 13.3 A.————-4) 21 . 155 . 48.7 Dip.3 . 50 Tabular.1 . 59 Difference.————-
Height of the eye, 15 feet, 3 inches.Parts of the horizon observed, NW. by N. and SE. by S.Depth of the water, 27 feet.
Barometer 29 . 68 inches.Thermometer { Air 84º{ Sea 83º
Latitude 38º. 50' north.Longitude 118º. 00' east.
The vessels in all parts of the horizon have an inverted image under them; this is very considerable, some having about a third of the sail, others only the hull.
No. 110.
July 28, 1817.—2.30. P.M.
Index uppermost. Instrument inverted.A + 6'. 35" B - 5'. 00"6 . 35 4 . 556 . 40 5 . 00———— ————-Mean 6 . 36.7 Mean 4 . 58.3 B.Mean + 6 . 36.7 A.————-4) 11 . 352 . 53.7 Dip.3 . 49 Tabular.55 Difference -————-
Height of the eye, 15 feet.Parts of the horizon observed, SE. and NW.
Thermometer { Air 64º{ Sea 59ºDepth of the sea, 222 feet.
Latitude 34º. 57' south.Longitude 20º. 15' east.
Cape Lagullus due north, distant 6 or 8 miles.
Calm all day; sky clear, and weather hazy.
On the 29th and 30th of July we were off the Cape, but the weather was so bad as to prevent any sights being taken.
[Transcriber's Note: The final column of each of the following tables is transcribed beneath the table.]
—+————+——+—————-+——-+———-+———-+——-+———-+———+———+| |Height Dip. |Diff.|Then. |Differ.| | | | || | of +—————-+—+—+—-+—-+—-+—-+ | | Long.|Sound-|No. Date. |eye.| Obs. |Tab.|+ |- |Sea|Air| + | - |Baro.| Lat. | East.| ings.|—+————+——+———+——+—+—+—-+—-+—-+—-+——-+———-+———+———+|1816. |f. i|' " |' "| "| "| º | º | | | | º ' | º ' | feet |18|June 16.|15 3|4 35.0|3 50|45| |83 |82-|1/2| |29.86| 5 11 S|106 3 | 60 || | | | | | | |1/2| | | | | | |19|June 16.|15 3|4 33.0|3 50|43| |83-|82 | 1-| |29.86| 5 05 S|106 10| 56 || | | | | | |1/2|sh.|1/2| | | | | || | | | | | | |{85| | | | | | |20|June 16.|14 0|3 59 |3 41|18| |84-|{sh|1/2| |29.83| 5 05 S|106 10| 48 || | | | | | |1/2|{95| | | | | | || | | | | | | |{su| | | | | | |21|June 16.|14 |4 01 |3 41|20| |83 |81 | 2 | |29.85| 5 05 S|106 10| 57 |22|June 27.|16 |4 21.0|3 56|25| |84 |82 | 2 | |29.81| 6 49 N|107 49| |23|June 28.|16 |4 22.2|3 56|26| |84 |82 | 2 | |29.80| 8 00 |108 10| |24|July 3. |16 |4 08.2|3 56|12| |84-|81 | 3-| |29.77|13 29 |112 59| || | | | | | |1/2| |1/2| | | | | |25|July 6. |16 |3 53 |3 56| | 3|84 |83-|1/4| |29.75|20 00 |114 | || | | | | | | |3/4| | | | | | |27|July 7. |15 3|4 3 |3 50|13| |85 |85 | | |29.79|21 11 |114 | |28|July 8. |14 6|3 49 |3 45| 4| |84-|82 | 2-| |29.72| | | || | | | | | |1/2| |1/2| | | | | |29|July 16.|15 3|3 27 |3 50| |23|79 |79 | | |29.75|24 37 |118 56| |30|July 21.|15 3|3 44 |3 50| | 6|77-|76 | 1-| |29.78|34 |124 | 270 || | | | | | |1/2| |1/2| | | | | |31|July 23.|15 3|3 49 |3 50| | 1|77 |82 | |5 |29.78|35 |124 | |32|July 23.|15 3|3 49 |3 50| | 1|77 |82 | |5 |29.78|35 |124 | |33|July 23.|15 3|3 44 |3 50| | 6|77 |82 | |5 |29.78|35 |124 | |35|July 27.|15 3|4 02 |3 50|12| |76 |76 | | |29.70|38 55 |118 50| 72 |36|July 27.|13 |3 35 |3 33| 2| |76 |76 | | |29.70|38 55 |118 50| 78 |37|July 28.|15 3|4 21 |3 50|31| |83 |84 | |1 |29.62|38 50 |118 00 20 |38|July 28.|15 3|4 06 |3 50|16| |83 |84 | |1 |29.62|38 50 |118 00| 20 |—+————+——+———+——+—+—+—-+—-+—-+—-+——-+———-+———+———+Remarks.—-+—————————————————————————————————————+18 |Weather hazy. The low land of Sumatra just visible. The land wind has ||been blowing gently for about four hours. |19 |East and west parts of the horizon observed. Coast of Sumatra just ||visible. Hazy. The land-wind dying away. |20 |The day has been extremely hot, and almost a calm. The sea-breeze not yet ||set in, only a few light flaws. |24 |A fresh breeze from WNW. The sun set in fiery dirty red clouds. Weather ||squally, with occasional showers of rain. Parts of the horizon observed ||east and west. |25 |Weather remarkably fine; sky clear; and a gentle breeze from the south. ||The sun set about five minutes after these observations were taken. ||Parts of the horizon observed east and west. |27 |The forenoon has been extremely hot and oppressive. A rolling swell from ||the SW. |28 |Parts of the horizon observed SSW and NNE, the first clear to seaward, || the other clear horizon, but the mainland of China behind it, and ||various islands on each side of the NNE line. |29 |Very hazy weather: sky fiery. |30 |Clear weather, with a light breeze from the eastward. Sun set behind a ||low range of dark clouds: sky in that quarter was unusually red. A long ||swell from the northward. |31 |Wind light from south; horizon uncommonly well defined and sharp; sky ||clear; and the sea perfectly smooth. These sights, and the two following, ||may be depended on, I think, within ten seconds. |32 |Circumstances similar to No. 31. |33 |Parts of the horizon observed were that immediately under the setting ||sun; viz. W 21º N, and the opposite E 21º S, the sun being about 4º high. ||Day has been remarkably clear, although the wind has been from the ||southward, which in these seas is said generally to bring fogs. |35 |Weather somewhat hazy; wind easterly. |36 |Wind easterly. |37 |These sights were taken while at anchor off the mouth of the Pei-ho. The ||fort of Tung-coo, on the south bank of the river, bearing W 50º N, distant||about four or five miles. |—-+—————————————————————————————————————+
—+————+——+—————-+———+————+———+——-+———+———+———+| |Height Dip. |Diff. |Then. |Differ.| | | | || | of +—————-+——+-+—-+—-+—-+—-+ | | Long.|Sound-|No. Date. |eye.| Obs. |Tab.| + |-|Sea|Air| + | - |Baro.| Lat.| East.| ings.|—+————+——+———+——+——+-+—-+—-+—-+—-+——-+———+———+———+|1816. |f. i|' " |' "|' " |"| º | º | | | | º ' | º ' | feet |39|July 28.|15 3|3 46 |3 50| |4|82 |83-| | 1-|29.61|38 50 |118 | 23 || | | | | | | |1/2| |1/2| | | | |40|July 29.|15 3|5 3 |3 50|1 13| |84 |81 | 3 | |29.60|38 50 |118 | 18 |41|July 29.|15 3|4 00.9|3 50| 10| |84 |83 | 1 | |29.58|38.50 |118 | 20 |42|Aug. 6. |15 3|5 09 |3 50|1 29| |80 |79 | 1 | |29.64|38 50 |118 | 22-|| | | | | | | | | | | | | | 1/2|43|Aug. 6. |15 6|4 54 |3 53|1 1| |81-|83-| | 2 |29.64|38 50 |118 | 29 || | | | | | |1/2|1/2| | | | | | |44|Aug. 6. |15 3|4 47 |3 50 57| |81-|83-| | 2 |29.64|38 50 |118 | 29 || | | | | | |1/2|1/2| | | | | | |45|Aug. 6. |15 3|4 59 |3 50|1 9| |81-|83-| | 1-|29.64|38 50 |118 | 29-|| | | | | | |1/2|1/2| |1/2| | | | 1/2|46|Aug. 6. | 3 9|2 39 |1 54| 45| |81 |84-| | 3-|29.62|38 50 |118 | 30 || | | | | | | |1/2| |1/2| | | | |47|Aug. 6. | 6 |3 26 |2 25|1 1| |81 |84-| | 3-|29.62|38 50 |118 | 30-|| | | | | | | |1/2| |1/2| | | | 1/2|48|Aug. 6. |15 3|4 59.2|3 50|1 9| |82 |80 | 2 | |29.59|38 50 |118 | 30 |49|Aug. 8. |15 3|5 47 |3 50|1 57| |78 |69-| 8-| |29.65|38 50 |118 | 26 || | | | | | | |1/2|1/2| | | | | |50|Aug. 8. |15 3|5 49 |3 59|1 59| |78 |69-| 8-| |29.65|38 50 |118 | 26 || | | | | | | |1/2|1/2| | | | | |51|Aug. 8. |15 3|5 47 |3 50|1 57| |77-|73 | 4-| |29.66|38 50 |118 | 24 || | | | | | |1/2| |1/2| | | | | |52|Aug. 9. |15 3|4 30.4|3 59| 40| |79-|75 | 4-| |29.72|38 50 |118 | || | | | | | |1/2| |1/2| | | | | |53|Aug.10. | 5 6|5 18.3|2 20|2 58| |77 |75 | 2 | |29.69|38 50 |118 | 5 |54|Aug.10. | 5 6|4 28.3|2 20|2 8| | | | | | |38 50 |117 55| |55|Aug.10. | 5 6|4 7 |2 20|1 47| |77 |75 | 2 | |29.69|38 50 |117 55| 12 |56|Aug.10. | 5 6|3 55 |2 20|1 35| |77 |75 | 2 | |29.69|38 50 |118 | 13 |—+————+——+———+——+——+-+—-+—-+—-+—-+——-+———+———+———+Remarks.—+—————————————————————————————————————-+39|The day has been exceedingly close with little wind. |40|There has been little wind this morning, after a very close night. |41|Nearly calm, there being only a very light air from the SE.—Day sultry. |42|Weather hazy; sky clear overhead; sea remarkably smooth; wind north. |43|Moderate breeze from SE by S; rather hazy; but the horizon sharp and ||distinct. And this together with the four following observations, may be ||taken as very accurate, every circumstance being most favourable. |45|Parts of the horizon observed E by N and W by S. |46|Parts of the horizon observed SSE and NNW. Wind SE. |47|Wind SE. |48|Parts of the horizon observed E and W. The day, which has been remarkably ||fine, has resumed towards sunset a wild, stormy aspect. Wind fresh at SE. |49|Parts of the horizon observed E by S and W by N. This morning unusually ||clear; so that when the sun's semi-diameter only was above the horizon, ||it was painful to look at him. The horizon has a rugged appearance. |50|Parts of the horizon observed NNE and SSW. Wind NNW, moderate. |51|Weather remarkably clear; horizon still rugged; wind NNW, moderate. |52|This morning cloudy, and looks rainy but the air seems clear. Parts of the ||horizon observed NW by W and SE by E. |53|Parts of the horizon observed N by E and S by W. Instrument readjusted. ||Wind WNW. |54|Parts of the horizon observed WSW and ENE. Wind NW. These observations ||were taken close to the low land, near the mouth of the Pei-ho. The night ||had been cold, and the morning was still keen; but unfortunately there ||was no thermometer in the boat; I suppose, however, that the air was ||about 66º At the time these sights were taken, I observed a vessel bearing ||N by W, the lower half of whose sail was inverted. |55|Wind NW. |56|Wind NW. Parts of the horizon observed NW and SE. During these ||observations,(53, 54, 55, 56) the vessels near the land had more or less ||an inverted image under them. |—+—————————————————————————————————————-+
—+————+——+—————-+———+————+———+——-+———+———+———+ | |Height Dip. |Diff. |Then. |Differ.| | | | | | | of +—————-+——+-+—-+—-+—-+—-+ | | Long.|Sound-| No. Date. |eye.| Obs. |Tab.| + |-|Sea|Air| + | - |Baro.| Lat.| East.| ings.| —+————+——+———+——+——+-+—-+—-+—-+—-+——-+———+———+———+ |1816. |f. i| ' " |' "|' " |"| º | º | | | | º ' | º ' | feet | 57|Aug. 10.|15 3| 5 37 |3 50|1 47| |78 |76-| 1-| |29.70|38 50 |118 | 26 | | | | | | | | |1/2|1/2| | | | | | 58|Aug. 10.|15 3| 6 6 |3 50|2 16| |83 |84 | | 1 |29.68|38 50 |118 | 27 | 59|Aug. 10.|15 3| 5 49 |3 50|1 59| |83 |84 | | 1 |29.68|38 50 |118 | 27 | 60|Aug. 11.|15 3| 5 3 |3 50|1 13| |79 |76 | 3 | |29.72|38 50 |118 | 26-| | | | | | | | | | | | | | | 1/2| 61|Aug. 11.|15 3| 5 13 |3 50|1 23| |80 |79 | 1 | |29.73|38 50 |118 | 24 | 62|Aug. 12.|16 | 4 52 |3 56| 56| |80 |79 | 1 | |29.79|38 50 |118 | 24 | 63|Aug. 12.|15 3| 4 35 |3 50| 45| |81 |79-| 1-| |29.77|38 36 |117 56| 29-| | | | | | | | |1/2|1/2| | | | | 1/2| 64|Aug. 13.|16 | 4 4 |3 56| 8| |79 |78 | 1 | |29.80|38 31 |118 09| 42 | 65|Aug. 13.|16 | 4 6 |3 56| 10| |79 |78 | 1 | |29.80|38 31 |118 09| 42 | 66|Aug. 13.|16 | 4 20 |3 56| 24| |81 |83 | | 2 |29.80|38 21 |118 04| 44 | 67|Aug. 14.|15 3| 4 30 |3 30|1 | |78 |79 | | 1 |29.71|38 30 |118 24| | 68|Aug. 14.|15 3| 4 25 |3 30| 55| |79-|80-| | 1 |29.70|38 30 |118 35| 50 | | | | | | | |1/2|1/2| | | | | | | 69|Aug. 15.|15 3| 4 39 |3 30|1 9| |80 |79 | 1 | |29.77|38 00 |118 35| 48 | 70|Aug. 15.|15 3| 4 53 |3 30|1 23| |81-|79 | 2-| |29.76|38 00 |118 54| 44 | | | | | | | |1/2| |1/2| | | | | | 71|Aug. 15.|15 3| 5 4 |3 30|1 34| |82 |79 | 3 | |29.70|37 54 |118 56| 40 | 72|Aug. 16.|15 3| 4 43 |3 30|1 13| |81 |76 | 5 | |29.17|37 38 |118 57| 39 | 73|Aug. 17.|15 3| 4 38 |3 30|1 8| |80 |79-|1/2| |29.73|37 21 |119 28| 30 | | | | | | | | |1/2| | | | | | | 74|Aug. 17.|14 | 4 29 |3 41| 48| |81-|81 |1/2| |29.75|37 19 |119 44| 33 | | | | | | | |1/2| | | | | | | | 75|Aug. 17.|15 3| 4 42 |3 30|1 12| |81-|82 | |1/2|29.70|37 21 |119 44| 30 | | | | | | | |1/2| | | | | | | | 76|Aug. 18.|16 | 4 39 |3 56| 43| |80 |77 | 3 | |29.76|37 29 |119 37| 48 | 77|Aug. 20.|16 | 4 20 |3 56| 24| |77 |72 | 5 | |29.85|37 50 |120 16| | 79|Aug. 21.|15 3| 4 42 |3 50| 52| |77 |71 | 6 | |29.80|37 52 |120 27| | 80|Aug. 21.| 4 | 2 37 |1 58| 39| |77 |79 | | 2 |29.80|37 52 |120 27| 60 | 81|Aug. 21.|15 3| 4 6 |3 50| 18| |77 |76 | 1 | |29.76| | | 60 | —+————+——+———+——+——+-+—-+—-+—-+—-+——-+———+———+———+ Remarks. —+—————————————————————————————————————-+ 57|Weather very clear. Parts of the horizon observed ENE and WSW. | 58|Parts of the horizon observed N by E and S by W. | 59|The vessels in all parts of the horizon have an inverted image under them; | |this is very considerable, some having about one-third of the sail, others | |only the hull. | 60|Light wind from SE. Sky cloudy, somewhat hazy; but the horizon sharp and | |unbroken. | 61|The inversion of the vessels as conspicuous as before. Parts of the | |horizon observed ESE and WNW. | 62|A light breeze from the SE. Cloudy and close.—N.B. Instrument readjusted. | 63|Part of the horizon observed N and S. | 64|Part of the horizon observed E and W. A moderate breeze from the SW. Clear | |overhead; hazy in the horizon. | 65|Parts of the horizon observed N and S. | 66|Wind SE. Sky clear, and the horizon sharp. | 67|Light breeze from ESE. Parts observed NE and SW. | 68|Light breeze at ESE. Cloudy, with a haze in the horizon. Parts observed NE | |and SW. | 69|Parts of the horizon observed E and W. Moderate breeze from ENE. | |Remarkably clear weather. | 70|Steady moderate breeze at ENE. Very clear. Horizon sharp, and well defined.| 72|Wind at East. Sky cloudy and rather hazy. | 73|Light wind at SW. Hazy weather. | 74|Light breeze from the Northward. Weather hazy. Parts of the horizon | |observed SE and NW. | 75|Parts of the horizon observed NE by E and SW by W. | 76|Moderate breeze from East. Parts of the horizon observed WNW and ESE. | 77|The wind has been blowing hard for two days from NE; this evening it has | |lulled, and the weather has cleared off: there remains however a high | |swell. | 79|Land-wind South. Fine clear morning. | 80|Parts of the horizon observed SW by S and NE by N. | 81|The inversions which were so conspicuous this morning have been entirely | |removed since the sea breeze set in. In some distant islands there is a | |slight inversion at the ends, but very trifling. | —+—————————————————————————————————————-+
—+————+——+—————-+———-+———-+———-+——-+———+———+———+| |Height Dip. |Diff. |Then. |Differ.| | | | || | of +—————-+——+—+—-+—-+—-+—-+ | | Long.|Sound-|No. Date. |eye.| Obs. |Tab.| + | -|Sea|Air| + | - |Baro.| Lat.| East.| ings.|—+————+——+———+——+——+—+—-+—-+—-+—-+——-+———+———+———+| 1816. |f. i| ' " |' "|' " | "| º | º | | | | º ' | º ' | feet |82|Aug. 21.|14 | 3 26 |3 41| |15|76 |75 | 1 | |29.74| | | |83|Aug. 21.|15 3| 3 29 |3 41| |12|76 |75 | 1 | |29.74| | | |84|Aug. 22.|16 | 3 52 |3 56| | 4|75 |76 | 1 | |29.80| | | |85|Sept. 4.|12 | 3 46 |3 25| 21| |81 |83 | | 2 |29.86| 36 10|126 30| 56 |86|Sept. 4.|12 | 3 42 |3 25| 17| |81 |83 | | 2 |29.86| 36 8|126 35| 56 |87|Sept. 5.|15 | 3 47 |3 49| | 2|79 |80 | | 1 |29.80| 35 40|126 17| |88|Sept. 7.|15 3| 3 33 |3 41| | 8|74 |80 | | 6 |29.84| 34 32|126 34| |89|Sept. 7.|15 3| 3 27 |3 41| |14|74 |80 | | 6 |29.84| 34 32|126 34| || 1817. | | | | | | | | | | | | | |90|Mar. 3. |14 | 3 57 |3 41| 16| |82 |84-| | 2-|29.74| 2 18|102 20| || | | | | | | |1/2| |1/2| | | | |91|Mar. 5. |14 8| 4 49 |3 46| 1 3| |83 |85 | | 2 |29.73| 3 40|100 35| |92|Mar. 5. |14 8| 4 50 |3 46| 1 4| |83 |85 | | 2 |29.73| | | 165 |93|Mar. 5. |14 8| 4 53 |3 46| 1 7| | | | | | | | | |94|Mar. 8. |14 8| 4 33 |3 46| 47| |84 |84 | | |29.86| 5 12|100 14| 120 |a)| | | | | | | | | | | | | | |94|Mar. 8. |14 8| 4 58 |3 46|1 12| |84 |82-| 1-| |29.86| | | 120 |b)| | | | | | | |1/2|1/2| | | | | |—+————+——+———+——+——+—+—-+—-+—-+—-+——-+———+———+———+Remarks.—+——————————————————————————————————————+82|This and the following were observed towards sunset; they exhibit a ||considerable degree of refraction above what is usual. The sights on this ||morning in the same place gave upwards of 1' greater dip. |83|All other circumstances the same as in No. 82. |84|Fresh breeze from SE, with a remarkably clear sky. The horizon uncommonly ||sharp. |85|Wind moderate from WNW. Clear weather. |86|All other circumstances the same as in No. 85. Parts of the horizon ||observed SSW and NNE. |87|Parts of the horizon observed WNW and ESE. |88|The morning has been exceedingly hot before the breezes set in from sea at ||11 A.M. |89|All other circumstances the same as in No. 89. |90|After a very hot day. |91|Parts of the horizon observed ESE and WNW. See further remarks under 93. ||Instruments readjusted. |92|Parts of the horizon observed NE and SW. All other circumstances as in No. ||91. |93|Parts of the horizon observed SSE and NNW. These three observations (Nos. ||91, 92, and 93) were made under the most favourable circumstances, and may ||be considered as shewing the accuracy which the instrument is capable of ||attaining. The sea was so perfectly smooth, that not the slightest motion ||could be detected. The horizon at all the parts observed was sharp, and ||better defined than I recollect to have seen it; and, what is not often the ||case, the opposite parts were alike in strength of light &c. The day has ||been hot, but not close, with a light breeze from the Southward. The dip is ||very great, but the observations were made with such care, that there can ||be no doubt of their accuracy. |94|There had been a light breeze from the North in the morning, but for an |a)|hour before these sights were taken it had been calm. |94|Nearly the same place as No. 94(a); but the other circumstances were |b)|changed, as the sea breeze at NW had set in about a quarter of an hour, ||whereas in the last instance it was calm. The above angles were taken with ||great care. The horizon sharp. Parts of the horizon observed NW and SE. |—+——————————————————————————————————————+
—-+————+——+—————-+———+———-+———-+——-+———+———+———+ | |Height Dip. |Diff. | Then. |Differ.| | | | | | | of +—————-+—+—-+—-+—-+—-+—-+ | | Long.|Sound-| No.| Date. |eye.| Obs. |Tab.| +| - |Sea|Air| + | - |Baro.| Lat.| East.| ings.| —-+————+——+———+——+—+—-+—-+—-+—-+—-+——-+———+———+———+ | 1817. |f. i|' " |' "| "|' "| º | º | | | | º ' | º ' | feet | 95|Mar. 19.|14 8| 3 51 |3 46| 5| |80 |82 | | 2 |29.84| | | | 96|Mar. 19.|14 8| 3 48 |3 46| 2| |80 |82 | | 2 |29.78|13 30 | 89 30| | 97|Mar. 20.|14 8| 3 48 |3 46| 2| |79-|82 | | 2-|29.83|14 30 | 89 15| | | | | | | | |1/2| | |1/2| | | | | 98|Mar. 21.|14 8| 3 48 |3 46| 2| |80 |82 | | 2 |29.84|15 00 | 89 00| | 99|Mar. 22.|14 8| 3 39 |3 46| | 7|79 |79-| |1/2|29.84|16 00 | 88 30| | | | | | | | | |1/2| | | | | | | 100|Mar. 23.|14 8| 3 47 |3 46| 1| |78-|79 | |1/2|29.80|17 00 | 88 00| | | | | | | | |1/2| | | | | | | | 101|Mar. 24.|14 8| 3 53 |3 46| 7| |78 |80 | | 2 |29.78|17 30 | 88 15| | | | | | | | | | | | | |South.| | | 102|July 22 |15 | 3 36 |3 49| | 13|71 |72 | | 1 | |34 0 | 26 | 400 | 103|July 24.|15 | 3 16 |3 49| | 33|59 |62 | | 3 | |34 25 | 24 56| 372 | 104|July 25.|15 | 3 36 |3 49| | 13|62 |63 | | 1 | |35 S | 23 45| 462 | 105|July 26.|15 | 3 30 |3 49| | 19|58 |60 | | 2 | |35 S | 23 | 462 | 106|July 26.|15 | 3 30 |3 40| | 19|60 |63 | | 3 | |34 52 | 22 23| 420 | 107|July 27.|15 | 2 55 |3 49| | 54|56 |59 | | 3 | |35 | 21 | 24 | 108|July 27.|15 | 2 47 |3 49| |1 2|56 |59 | | 3 | |35 | 21 | 240 | 109|July 28.|15 | 3 17 |3 19| | 32|58 |64 | | 6 | |34 58 | 20 15| | 110|July 28.|15 | 2 54 |3 49| | 55|59 |64 | | 5 | |34 57 | 20 15| 222 | —-+————+——+———+——+—+—-+—-+—-+—-+—-+——-+———+———+———+ Remarks. —-+—————————————————————————————————————-+ 95|The wind steady and moderate at NE; atmosphere clear; horizon well defined;| |a long swell from SW. This swell, which was not high, produced an obvious | |effect on the dip angle, as observed, the two horizons alternately | |separating and overlapping; this change was however so slight that I have | |not been able to measure it. | 96|All other circumstances as in Nov. 95. Parts of the horizon observed NE | |and SW. | 97|During the night there has been a light breeze from the East; at this | |moment it is freshening up a little. The atmosphere is clear; horizon | |sharp; a long low swell from SW, as yesterday. | 98|It has been calm, or nearly so, during the night; occasionally a light air | |from SE and S. All circumstances favorable. | 99|During the night there has been a light wind from SW. The weather is more | |hazy than when the wind was from the Eastward, and the horizon not so | |distinctly marked; but the above sights are good. There is still a swell | |from SW, which causes some little uncertainty as to the exact moment of | |taking the angle. | 100|In the night there has been a light breeze from W by S. Weather hazy; but | |the horizon sharper than yesterday morning. | 101|During the night almost calm; just now a light air from the NE. Parts of | |the horizon observed NE and SW. | 102|Fine fair clear weather, but with so high a swell as to render the | |observation difficult. Wind light from N, after having been blowing fresh. | 103|Light breezes from the North-eastward; smooth water, and a clear cool air; | |hazy about the land. The distance from the South cost of Africa was about | |8 or 9 leagues. All circumstances seem favourable. No current; we have | |probably been too near shore for it. | 104|Light breeze from the SW, with a long swell. Hazy weather. The wind has | |been from the West for 24 hours; at first blowing hard, but latterly | |moderate, the current setting us to the SW about a mile an hour. A very | |heavy dew falling this evening. Parts of the horizon observed East and | |West. | 105|Moderate breeze from the NE; air hazy; long high swell from the Westward. | |From observations by stars and chronometers, it has been ascertained that | |there is not the least current. Distance from the South coast of Africa | |about 50 miles. A high range in sight to the Northward. Parts of the | |horizon observed North and South. | 106|A light breeze from the NE; air hazy; a long swell from the Westward. | |About 50 miles distant from the land. | 107|The wind has been moderate from the land all night; air hazy; weather raw; | |a very heavy dew falling all night. The land in sight to the Northward, | |distant about 40 miles, is inverted from one end to the other. | 108|Parts of the horizon observed NNE and SSW. All other circumstances as in | |No. 107. | 109|Parts of the horizon observed NE and SW. Cape Lagullas North 2 or 3 | |leagues. A light breeze from the Eastward. Air hazy. | 110|Parts of the horizon observed SE and NW. Cape Lagullas due North, distant | |6 or 8 miles. Calm all day; sky clear; and weather hazy. | | | |N.B. On the 29th and 30th of July we were off the Cape, but the weather | |was so bad as to prevent any sights being taken. | —-+—————————————————————————————————————-+