Navigators, when they met with them, to think that they were caused by the nearness of some large Tracks of Land: but I rather think they were owing to another Cause. It hath been found both by the Dolphin and us that the trade winds in those parts of this Sea doth not extend further to the Southward than 20°, and without which we generally meet with a wind from the westward. Now, is it not reasonable to suppose that when these winds blow strong they must encroach upon and drive back the Easterly winds as to cause the variable winds and S.-Westerly swells I have been speaking of? It is well known that the Trade winds blow but faint for some distance within their limits, and are therefore easily stopt by a wind from the Contrary direction. It is likewise known that these limits are subject to vary several degrees, not only at different seasons of the Year, but at one and the same season. Another reason why I think that these S.W. winds are not caused by the nearness of any large Track of land, is in their being always accompanied with a large swell from the same Quarter, and we find a much greater surf beating upon the Shores of the S.W. sides of the Islands situated just within the Limits of the Trade winds than upon any other part of them.
The tides are perhaps as inconsiderable in these Seas as in any part of the world. A S. or S. by W. moon makes high water in Royal Bay, but the water does not rise upon a perpendicular above 10 or 12 inches, except on some very Extraordinary occasions.
The variation of the Compass I found to be 4° 46′ Easterly, this being the mean result of a great number of Trials made by 4 of Dr. 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. This will in a great measure account for the seeming errors that may, upon a nice examination, appear to have been made in observing the Variation inserted in the Course of this Journal. This variableness in Magnetick Needles I have many times and in many places experienced both ashore and on board of Ships, and I do not remember of ever finding two Needles that would agree exactly together at one and the same time and place, but I have often found the same Needle agree with itself for several Trials made immediately one after another.[1] However, all this is of no sort of consequence to Navigation, as the Variation of the Compass can always be found to a degree of accuracy more than sufficient for all nautical Purposes.
- ↑ These discrepancies result from imperfections in the suspension and mounting of the needles, and are only absent in instruments too delicate for ordinary sea service.
