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the life of his father Christopher, to be some of the reasons which moved the said Christopher Columbus to attempt the discovery of the West Indies. The things mentioned by them are, 1st. A piece of wood ingeniously wrought, but not with iron, taken up by Martin Vicenzo, a Portuguese pilot, 450 leagues at sea, off Cape St. Vincent, after a west wind of many days. 2dly. Another piece of wood, like the former, taken up by Pietro Correa, on the island of Porto Santo, after the like winds. 3dly. Very large canes, much beyond any growing in those parts. 4thly. Some of the inhabitants of the Azores observed, and told him, that west winds brought pines to these islands, especially Fayal and Graciosa, which are not found growing in those parts; and that on another of those islands, viz. Flores, was cast on shore two men's bodies, with larger faces, and different aspects, from Christians; and that at Capo della Verga were once seen two canoes or barks with cabins, which were believed to be forced to sea, when accidentally they had been going from one island to another,
On the Structure of the Internal Parts of Fish. By Dr.
CHARLES PREston.- [1696-7.] The principal difference between fish and other animals is their want of lungs and respiration; whereas all other animals have lungs, both terrestrial, volant, and amphibious ; and in insects, the several tracheæ, that are spread throughout the whole body, serve them instead of lungs. And yet it is necessary that something should supply this in fishes, which
may have the same effect on their blood, as the air has upon qurs, by entering into our lungs, viz. to divide and dissolve it, and render it fit for circulation. Now we find no part in fish more proper to produce this effect than the bronchia, that lie like so many leaves over each other under their gills; for they receive the water in by the mouth, and return it by the gills; or receiving it in by the gills, they throw it out by the mouth.
It is agreed upon by all, that the water contains something that produces this effect; and this seems most probably to be the air contained in the water, that dissolves the blood in the bronchịa of fish, as well as it does that in the lungs of all other animals. That there is air in all water, cannot be doubted, after the experiment of M. Marolle. He set a vessel of water over the fire, so as to drive out the air from it: this water he put into the air-pump, to extract the air from it; and after that filled a phial with it, within two or
three fingers of the top, which space he left only full of air, and stopped the phial well ; and by shaking it, the water imbibed the air, so as to rise up and quite fill the phial. It
may be objected, that if the air in the water were the cause of this effect, the fish would live in the open air. I shall only reply to this, that fish have their blood less hot than ours, so that the natural heat of our blood would in them be a fever and mortal ; hence we need not wonder they cannot live in the air ; for the nitre of the pure air is in too great a quantity, and too subtle, so as it dissolves their blood too much, and makes it too fuid ; whereas the nitre in the water is more gross, and in less proportion ; whence it gives their blood only a fluidity requisite to keep it in its natural state. To prove that it is in the bronchia that this division is performed, we need only observe their extraordinary redness above any other part of the body, a proof that the blood is there more divided : fish are also found to die in water frozen over, which happens plainly from their communication with the external air being hindered by the ice.
The heart of a fish is different from that of other animals in its having only one ventricle ; for it has only the vena cava and the aorta that open into it, having no lungs; so that by the aorta the blood comes out of the heart, which is branched into. a thousand capillaries over the bronchia, and is afterwards reunited; which re-union is made under the basis of the cranium; and because the blood, when once there, has no need of being forced higher upwards, they have no occasion for a second ventricle for that purpose, as terrestrial animals have." The re-union of these capillaries of the bronchia being made, they form two large trunks, of which one proceeds towards the head, and the other towards the lower parts.
Fish have a diaphragm, but not for the same purpose as in other animals that breathe; it is always straight and tense, and perpendicular on the vertebræ. Their stomach is membranous: for fish swallow down other small fish whole, and sometimes earth ; therefore it is necessary to have a power of contracting itself forcibly to break in pieces its contents. Their intestines make several great windings, a sign the fermentation is but slow in them, which is made up by their great length.
Fish have on the vertebræ of the loins a bladder, very large in proportion to their bulk, which serves, by dilating or compressing itself, to render the fish lighter or heavier, as occasion requires, for swimming. And if this be by any means burst, so that it cannot be extended, the fish can no more raise itself in the water, but keeps continually at the bottom. The fins and tail assist them in their passage through the water : but it is this dilatation of the air in the bladder that makes them capable of swimming, after the same manner as the dilating of the lungs and thorax of a man bears him up in the water. Flat fish, such as soles, have none of this bladder; for they are able, by reason of their breadth, to sustain themselves in the water. Craw-fish and other shell-fish want it likewise, for the most part, for they creep only at the bottom of the water, but there are many fish that have them double..
The Theory of the Tides, extracted from Mr. Isaac Newton's
Treatise, entitled, Philosophiæ Naturalis Principia Mathematica. By Mr. Edmund HALLEY.—[1696-7.]
The sole principle on which this author (Mr. Newton) proceeds to explain most of the great and surprising appearances of nature, is no other than that of gravity, by which all bodies in the earth have a tendency towards its centre. From this principle, as a necessary consequence, follows the spherical figure of the earth and sea, and of all the other celestial bodies; and though the tenacity and firmness of the solid parts support the inequalities of the land above the level, yet the fluids, pressing equally, and easily yielding to each other, soon restore the equilibrium, if disturbed, and maintain the exact figure of the globe.
Now this force, of the descent of bodies towards the centre, is not in all places alike, but is still less and less, as the distance of the centre increases, and in this book it is demonstrated, that this force decreases as the square of the distance increases, that is, the weight of bodies and the force of their fall is less, in parts more removed from the centre, in the proportion of the squares of the distance. Thus a ton weight on the surface of the earth, if it were raised to the height of 4000 miles, which I suppose the semidiameter of the earth, would weigh but ¢ of a ton, or 500 weight; if to 12,000 miles, or three semidiameters from the surface, that is four from the centre, it would weigh but to part of the weight on the surface, or 100 and 1 ; so that it would be as easy for the strength of a man at that height to carry a ton weight as here on the surface 14 cwt. And in the same proportion does the velocities of the fall of bodies decrease ; for whereas on the surface of the earth all things fall 16 feet in a second, at one semidiameter above the surface this fall is but four feet, and at three semidiameters, or four from the centre, it is but 16 of the fall at the surface, or but one foot in a second ; and at greater distances, both the weight and fall become very small; yet at all given distances they are still something, though the effect become insensible. At the distance of the moon, which I will suppose 60 semidiameters of the earth, 3600 pounds weigh only one pound, and the fall of bodies is but also of a foot in a second, or 16 feet in a minute ; that is, a body so far off would descend in a minute no more than the same at the surface of the earth descends in a second of time.
This law of the decrease of gravity being demonstratively proved, the author enquires into the necessary consequences of this supposition ; by which he finds the genuine cause of the several appearances in the theory of the moon and planets, and discovers the hitherto unknown laws of the motion of comets, and of the ebbing and flowing of the sea.
Now, the theory of the motion of the primary planets is here shown to be nothing but the contemplation of the curve lines, which bodies projected with a given velocity, in a given direction, and at the same time drawn towards the sun by its gravitating power, would describe. Or which is the same, that the orbits of the planets are such curve lines as a shot from a gun describes in the air, being thrown according to the direction of the piece, but bent into a crooked line by the supervening tendency towards the earth's centre; and the planets being supposed to be projected with a given force, and attracted towards the sun, after the aforesaid manner, are here proved to describe such figures as answer exactly to all that the industry of this and the last age has observed in the planetary motions. So that it appears, that there is no need of solid orbs and intelligences, as the ancients imagined ; nor yet of vortices or whirlpools of the celestial matter, as Descartes supposes ; but the whole affair is simply and mechanically performed, on the sole supposition of a gravitation towards the sun.
All the surprising phenomena of the flux and reflux of the sea are, in like manner, shown to proceed from the same principle. If the earth were alone, that is to say, not affected by the actions of the sun and moon, it is not to be doubted but the ocean, being equally pressed by the force of gravity Lowards the centre, would continue in a perfect stagnation, always at the same height, without ever ebbing or flowing ; but it being here demonstrated, that the sun and moon have a like principle of gravitation towards their centres, and that the earth is within the activity of their attractions, it will plainly follow, that the equality of the pressure of gravity towards the centre will thereby be disturbed ; and though the smallness of these forces, in respect of the gravitation towards the earth's centre, renders them altogether imperceptible by any experiments we can devise, yet the ocean, being fluid, and yielding to the least force, by its rising shows where it is less pressed, and where it is more pressed by its sinking. Now if we suppose the force of the moon's attraction to decrease, as the square of the distance from its centre increases, as in the earth and other celestial bodies, we shall find that where the moon is perpendicularly either above or below the horizon, in zenith or nadir, there the force of gravity is most of all diminished, and, consequently, that there the ocean must necessarily swell by the coming in of the water from those parts where the pressure is greatest, viz. in those places where the moon is near the horizon. This rightly understood, it plainly follows, that the sea, which otherwise would be spherical, by the pressure of the moon must form itself into a spheroidal or oval figure, whose longest diameter is where the moon is vertical, and shortest where she is in the horizon ; and that the moon shifting her position as she turns round the earth once a day, this oval of water shifts with her, occasioning thereby the two floods and ebbs observable in each 25 hours.
And this may suffice as to the general cause of the tides. It remains now to show how naturally this motion accounts for all the particulars that have been observed about them ; so that there can be no room left to doubt but that this is the true cause of them. The spring-tides at the new and full moons,
and neap-tides at the quarters, are occasioned by the attractive force of the sun in the new and full, conspiring with the attraction of the moon, and producing a tide by their united forces: whereas in the quarters, the sun raises the water where the moon depresses it, and the contrary ; so as the tides are made only by the difference of their attractions. That the force of the sun is no greater in this case, proceeds from the very small proportion the semidiameter of the earth bears to the vast distance of the sun.
It is also observed that, cæteris paribus, the equinoctial spring-tides in March and September, or near them, are the highest, and the neap-tides the lowest; which proceeds from the greater agitation of the waters, when the Auid spheroid revolves about a greater circle of the earth than when it turns about in a smaller circle ; it being plain, that if the moon were constituted in the pole, and stood there, that the