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the island of St. Thomas; and this is the constant practice of all the Guinea ships.

All ships departing from Guinea for Europe, their direct course is northward; but on this course they cannot go, because the coast being nearly east and west, the land is to the northward, therefore as the winds on this coast are generally between the S. and W. S. W. they are obliged to steer S. S. E. or S. and with these courses they run off the shore; but in so doing they always find the wind more and more contrary, so that when near the shore they can lie south; at a great distance they can make no better than S. E. and afterwards E. S. E. with which courses they generally fetch the island of St. Thomas, and Cape Lopez, where finding the winds to the eastward of the south, they sail westerly with it, till coming to the latitude of four degrees south, where they find the S. E. wind blowing perpetually.

On account of these general winds, all those that use the WestIndia trade, even those bound to Virginia, reckon it their best course to get as soon as they can to the southward, that so they may be certain of a fair and fresh gale to run before it to the westward; and for the same reason those homeward bound from America endeavour to gain the latitude of 30°, where they first find the wind begin to be variable, though the most ordinary winds in the North Atlantic ocean come between the south and west.

Between the southern lats. of 10 and 20° in the Indian ocean, the general trade-wind, about S. E. by S. is found to blow all the year round in the same manner as in the like lats. in the Ethiopic ocean: and during the six months, from May to December, these winds reach to within 22 of the equator; but during the other six months, from November to June, a N. W. wind blows in the tract lying between the 3d and 10th degrees of southern lat. in the meridian of the north end of Madagascar; and between the 2d and 12th degrees of south lat. near the long. of Sumatra and Java.

In the tract between Sumatra and the African coast, and from 3° of S. lat. quite northward to the Asiatic coast, including the Arabian sea and the Gulf of Bengal, the monsoons blow from September to April on the N. E. and from March to October on the S. W. In the former half-year, the wind is more steady and gentle, and the weather clearer, than in the latter six months: and the wind is more strong and steady in the Arabian sea than in the Gulf of Bengal.

Between the island of Madagascar and the coast of Africa, and bence northward as far as the equator, there is a tract wherein, from April to October, there is a constant fresh S. S. W. wind, which, to the northward, changes into the W. S. W. wind blowing, at that time, in the Arabian sea.

To the eastward of Sumatra and Malacca, on the north of the equator, and along the coasts of Cambodia and China, quite through the Philippines, as far as Japan, the monsoons blow northerly and southerly; the northern setting in about October or No

vember, and the southern about May. These winds are not quite so certain as those in the Arabian sea.

Between Sumatra and Java to the west, and New Guinea to the east, the same northerly and southerly winds are observed: but the first half-year the monsoons incline to the N. W. and the latter to the S. E. These winds begin a month or six weeks after those in the Chinese seas set in, and are quite as variable.

These contrary winds do not shift from one point to its opposite all at once; in some places the time of the change is attended with calms; in others by variable winds; and it often happens on the shores of Coromandel and China, towards the end of the monsoon, that there are most violent storms, greatly resembling the hurricanes in the West Indies, wherein the wind is so vastly strong, that hardly any thing can resist its force.

All navigation in the Indian ocean must necessarily be regulated by those winds: for if mariners should delay their voyages till the contrary monsoon begins, they must either sail back, or go into harbour, and wait for the changing of the trade-winds.

Vapours rising from the sea, and by the wind carried over low lands to the ridges of mountains, and compelled to mount up with the stream of the air to the tops, where the water presently precipitates, gliding down by the chinks and cliffs of the stones, and part of the water entering into the caverns of hills, and gathering into basons, which being once filled begin to run over, and form subterraneous passages through the earth, breaking out in springs by the sides of hills; several of those meeting together form a rivulet; several of these rivulets meeting together make a river. This, together with what is incorporated into vegetables, renders it impossible for all the water evaporated from the sea to return to it again. Hence the evaporations arising from the Mediterranean are such, that notwithstanding there are nine capital rivers, which empty themselves into it, beside smaller ones, there is a constant current running through the Straits of Gibraltar from the Atlantic ocean to make up the deficiency. R. Mean, M. D. and F. R. S. observes, 1. That some diseases are probably the effects of the influence of the heavenly bodies. 2. That the most windy seasons of the year are about the vernal and autumnal equinoxes. 3. All the changes we have enumerated in the atmosphere do fall out at the same times when those happen in the ocean; and, as both the waters of the sea and the air of our earth or fluids are subject in a great measure to the same laws of motion, so that natural effects of the same kind are owing to the same causes. 4. The alteration made by the sun and moon in the atmosphere must thereby have influence on the animal body. 5. The elasticity of the air is of great moment, and it is reciprocally as the pressure, so that the incumbent weight being diminished by the attraction, the air underneath will be much expanded; these, and such-like causes, will make the tides in the air to be much greater than those of the ocean; and there is no doubt to be made, but that the same infinitely wise Being, who contrive

the flux and reflux of the seas, to secure that vast collection of waters from staguation and corruption, has ordered this ebb and flood of the air of our atmosphere with the like good design; that is, to preserve it sweet, and a brisk temper of this fluid so necessary to life, by a continual circulation. 6. Two contrary winds blowing towards the same place, may accumulate the air there, so as to increase the height and the weight of the incumbent cylinder; in like manner the direction of two winds may be such, as meeting at certain angles, may keep the gravity of the air in a middle state; but if the wind blows different ways from the same place (which may be occasioned by thunder and lightning) the height and weight of the air may be much decreased. 7. The changes in our atmosphere at high-water, new and full moon, the equinoxes, &c. must occasion alterations in all animal bodies, for all living creatures require air of a determined gravity to perform respiration easily; for it is by its weight that this fluid insinuates itself into the cavity of the breast and lungs: by a slow circulation the secretion of the spirits is diminished; and by the want of the force of elasticity and gravity, the juices begin to ferment, change the union of their parts, break their canals, and diseases follow.

Besides the above causes, the atmosphere may be put in motion by the elastic vapours forced from the bowels of the earth by subterraneous heats, and condensed by whatever causes in the atmosphere. A mixture of effluvia in different qualities in the air may, by rarefaction, fermentation, &c. produce winds and other effects like those resulting from the combination of some chemical liquors; and that such things happen, we are assured from the nature of thunder, lightning, and ineteors. From the eruption of volcanoes and earthquakes in distant places, wind may be propagated to remoter countries. The divided or united forces of the other planets, and of the comets, may variously disturb the influence of the sun, the moon, &c. We know that there happen violent tempests in the upper region of the air, when we below enjoy a calm; and how nany ridges of mountains there are on our globe which interrupt and check the propagation of the winds, so that it is no wonder that the phenomena we have ascribed to the action of the sun and moon, are not always constant and uniform, and that every effect does not hereupon follow; which were there no other powers in nature able to alter the influence of, this might, in a very regular and uniform manner, be expected from it.

That the rarefied air ascends is sufficiently demonstrated by the aerostatic globe, or air-balloon, lately invented: this is a globe made of silk, or other light stuff, made air-tight with gum; which, being filled with inflammable or rarefied air, will, when let loose, ascend, until it comes to that part of the atmosphere that is nearly as light as the air within it, where it will contine some time.

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OF TIDE S.

A TIDE is that motion of the water in the seas and rivers, by which they regularly rise and fall: the general cause of which was discovered by Sir ISAAC NEWTON, and is deduced from the following considerations:-Daily experience shows, that all bodies, when thrown upwards from the earth, fall down to its surface in perpendicular lines; and as lines perpendicular to the surface of any sphere tend towards its centre, the lines, along which all heavy bodies fall, must be directed towards the earth's

centre.

As bodies appear to fall by their weight or gravity, the law, by which they descend, is called the law of gravitation: and as a magnet or loadstone will draw small portions of iron or steel, and as a piece of glass, amber, or sealing-wax, when warmed by rubbing, will draw small bits of paper, and other light substances, the law, by which such bodies fly to those which draw them, is called the law of attraction. Hence it is not improper to say, that bodies, when falling by their gravity towards the earth, are attracted by the earth; and therefore the words gravitation and attraction may, respecting the earth, be used indifferently, as by them is only meant that power, or law, by which all bodies tend towards its centre.

Sir ISAAC discovered, by a great number of observations, that, this law of gravitation or attraction was universally diffused. throughout the solar system; and that the regular motions, observed among the heavenly bodies, were governed by it; so that the earth and moon attract each other, and both of them are attracted by the sun. He also discovered, that the force of attraction, mutually exerted by these bodies, was lessened as the distance increased, in proportion to the squares of those distances; that is, the power of attraction, at double the distance, was four times less; at triple the distance, nine times less; at quadruple the distance, sixteen times less, and so on.

As the earth is attracted by the sun and moon, it follows, that all the parts of the earth will not gravitate towards its centre in he same manner as they would do, if those parts were not affected y such attractions. And it is evident, that were the earth enirely free from such actions of the sun and moon, the ocean, beng on all sides equally inclined towards its centre by the force of gravity, would continue in a perfect stagnant state, without ever bbing or flowing. But, as the case is otherwise, the water in he ocean must needs rise higher in those places where the sun and noon diminish its gravity, or where they have the greatest at

raction.

As the force of gravity must be diminished most in those parts of the earth to which the moon is nearest, or in the zenith, because her attraction will there be most powerful; therefore the waters, in such places, will rise higher, and it will in them be full sea or high-water. The parts of the earth directly under the moon, and also those in the nadir, viz. such places as are diametrically opposite to those where the moon is in the zenith, will have high-water at the same time. For either half of the earth would gravitate equally towards the other half, were they superfluous free from all attraction. But by the action of the moon, the gravitation of one half of the earth towards its centre is diminished, and that of the other increased. In the half-earth next the moon, the parts die rectly under her being most attracted, and consequently their gra vitation towards the earth's centre most diminished, the waters in these parts must be higher than in any other part of this half-earth. And in the half-earth farthest from the moon, the parts in the nadir being less attracted by her than those which are nearer, gravitate towards the earth's centre, and consequently, the waters in those parts must be higher than they are in any other part of this half-earth.

Those parts of the earth where the moon appears in the horizon, or is 90 degrees distant from the zenith and nadir, will have their lowest waters. For as the waters in the zenith and nadir rise at the same time, the adjacent waters will press towards those places to restore the equilibrium; and to supply the places of these, others will move the same way, and so on to 90° distant from the said zenith and nadir: consequently the waters, in those places where the moon appears in the horizon, will have most liberty to descend towards the centre; and therefore they will, in such places, be the lowest. Hence it plainly follows, that the ocean, if it covered the surface of the earth, would put on a spheroidal, or egg-like figure, in which the longest diameter would pass through the place where the moon is vertical; and the shortest where she is in the horizon. And as the moon apparently shifts her position from east to west in going round the earth every day, the long diameter of the spheroid, following that motion, would occasion the two floods and ebbs in about every 25 hours, which is about the length of a lunar day, or the time spent between the moon's leaving the meridian of any place, and her coming to it again. Hence the greater the moon's meridian altitude is at any place,the greater will those tides be which happen when she is above the horizon; and the greater her meridian depression is, the greater will those tides be, which happen when she is below the horizon. The summer day, and the winter night, tides, have a tendency to be the highest; because the sun's summer elevation, and his winter depression, are greatest: this is more especially to be observed when the moon has north declination in summer and south declination in winter.

The time of high-water is not precisely at the time of the moon's

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