A Relation of Some Extraordinary Tydes in the West-Isles of Scotland, as It Was Communicated by Sr. Robert Moray

PHILOSOPHICAL TRANSACTIONS. Monday, June 5, 1665. The Contents. A Relation of some extraordinary Tydes in the West-Isles of Scotland, by Sr. Robert Moray. The judgment of Monsieur Auzout, touching the Apertures of Object-glasses, and their proportions in respect of the several lengths of Telescopes; together with a Table thereof. Considerations of the same Person upon Mr. Hook's New Engine for grinding of Optick-glasses. Mr. Hook's Thoughts thereupon. Of a means to illuminate an Object in what proportion one pleaseth; and of the distances, that are requisite to burn Bodies by the Sun. A further accompt by Monsieur Auzout of Signior Campani's Book, and Performances about Optick-Glasses. Campani's Answer thereunto; and Mr. Auzout's Animadversions upon that Answer. An accompt of Mr. Lower's newly published Vindication of Dr. Willis's Diatriba de Febribus. A Relation of some extraordinary Tydes in the West-Isles of Scotland, as it was communicated by Sr. Robert Moray. In that Tract of Isles, on the West of Scotland, called by the Inhabitants, the Long-Island, as being about 100 miles long from North to South, there is a multitude of small Islands, situated in a Fretum, or Frith, that passes between the Island of Eust, and the Harris; amongst which, there is one called Berneray, some three miles long, and more. more than a mile broad, the length running from East to West, as the Firth lies. At the East end of this Island, where I stayed some 16 or 17 days, I observed a very strange Reciprocation of the Flux and Reflux of the Sea, and heard of another, no less remarkable. Upon the West side of the Long Island, the Tides, which came from the South-west, run along the Coast, Northward; so that during the ordinary course of the Tides, the Flood runs East in the Firth, where Berneray lies, and the Ebb West. And thus the Sea ebbs and flows orderly, some 4 days before the full Moon, and change, and as long after (the ordinary Spring-tides rising some 14 or 15 foot upright, and all the rest proportionably, as in other places.) But afterwards, some 4 days before the Quarter moons, and as long after, there is constantly a great and singular variation. For then, (a Southerly Moon making there the full Sea,) the course of the Tide being Eastward, when it begins to flow, which is about 9½ of the Clock, not only continues so till about 3½ in the afternoon, that it be high water, but, after it begins to ebb, the Current runs on still Eastward, during the whole Ebbs so that it runs Eastward 12 hours together, that is, all day long, from about 9½ in the morning, til about 9½ at night. But then, when the night-Tide begins to flow, the Current turns, and runs Westward all night, during both Floud & Ebb, for some 12 hours more, as it did Eastward the day before. And thus the Reciprocations continue, one Floud and Ebb, running 12 hours Eastward, and another twelve hours Westward, till 4 days before the New and Full Moons; and then they resume their ordinary regular course as before, running East, during the six hours of Floud, and West, during the six of Ebb. And this I observed curiously, during my abode upon the place, which was in the Moneth of August, as I remember. But the Gentleman, to whom the Island belongs at present, and divers of his Brothers and Friends, knowing and discreet persons, and expert in all such parts of Sea-matters, as other Islanders commonly are, though I shrewdly suspected their skill in Tides, when I had not yet seen what they told me, and I have now related of these irregular Courses of the Tides, did most confidently assure me, and so did every body I spake with about about it, that there is yet another irregularity in the Tides, which never fails, and is no less extraordinary, than what I have been mentioning: which is, That, whereas between the Vernal and Autumnal Equinoxes, that is, for six Moneths together, the Course of irregular Tides about the Quartermoons, is, to run all day, that is, twelve hours, as from about $9\frac{1}{2}$ to $9\frac{5}{6}$, $10\frac{1}{4}$ to $10\frac{3}{4}$ &c. Eastward, and all night, that is, twelve hours more, Westward: during the other six Moneths, from the Autumnal to the Vernal Equinox, the Current runs all day Westward, and all Night Eastward. Of this, though I had not the opportunity to be an Eye-witness, as of the other, yet I do not at all doubt, having received so credible Information of it. To penetrate into the Causes of these strange Reciprocations of the Tides, would require exact descriptions of the Situation, Shape, and Extent of every piece of the adjacent Coasts of Eust and Harris; the Rocks, Sands, Shelves, Promontories, Bays, Lakes, Depths, and other Circumstances, which I cannot now set down with any certainty, or accurateness; seeing, they are to be found in no Map, neither had I any opportunity to survey them; nor do they now occur to my Memory, as they did some years ago, when upon occasion I ventured to make a Map of this whole Frith of Berneray, which not having copied, I cannot adventure to beat it out again. Monsieur Auzout's Judgment touching the Apertures of Object-Glasses, and their Proportions, in respect of the several Lengths of Telescopes. This Author, observing in a small French Tract lately written by him to a Countryman of his, Monsieur L' Abbé Charles; That great Optick Glasses have almost never as great an Aperture as the small ones, in proportion to what they Magnifie, and that therefore they must be more dim; takes occasion to inform