and the fouthern about May. These winds are not quite fo certain as those in the Arabian sea.

Between Sumatra and Java to the weft, and New Guinea to the eaft, the fame northerly and foutherly winds are obferved; but the first half year the monfoons incline to the N. W. and the latter to the S. E. These winds begin a month or fix weeks after thofe in the Chinese seas fet in, and are quite as variable.

These contrary winds do not fhift from one point to its oppofite all at once: in fome places the time of the change is attended with calms; in others by variable winds; and it often happens on the fhores of Coromandel and China, towards the end of the monfoon, that there are moft violent ftorms, greatly refembling the hurricanes in the West Indies, wherein the wind is fo vaftly ftrong, that hardly any thing can refift its force.

All navigation in the Indian ocean muft neceffarily be regulated by those winds; for if mariners fhould delay their voyages till the contrary monfoon begins, they muft either fail back, or go into harbour, and wait for the changing of the trade winds.

Vapours rifing from the fea, and by the wind carried over low lands to the ridges of mountains, and compelled to mount up with the ftream of the air to the tops, where the water prefently precipitates, gliding down by the chinks and cliffs of the ftones, and part of the water entering into the caverns of hills, and gathering into bafons, which being once filled begin to run over, and form fubterraneous paffages through the earth, breaking out in fprings by the fides of hills; feveral 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 impoffible for all the water evaporated from the fea to return to it again. Hence the evaporations arifing from the Mediterranean are fuch, that notwithstanding there are nine capital rivers, which empty themselves into it, befide fmaller ones, there is a conftant current running through the Straits of Gibraltar from the Atlantic ocean, to make up the deficiency. R. Mean, M. D. and F. R. S. obferves, 1. That fome 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 fame times when those happen in the ocean; and, as both the waters of the fea and the air of our earth or fluids are fubject in a great me fure to the fame laws of motion, fo that natural effects of the fame kind are owing to the fame caufes. 4. The alteration made by the fun and moon in the atmosphere must thereby have influence on the animal body. 5. The elafticity of the air is of great moment, and it is reciprocally as the preffure, fo that the incumbent weight being diminished by the attraction, the air underneath will be much expanded; thefe, and fuch like caufes, 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 fame infinitely wife Being, who contrived

the flux and reflux of the feas, to fecure that vaft collection of waters from ftagnation and corruption, has ordered this ebb and flood of the air of our atmosphere with the like good defign; that is, to preserve it fweet, and a brifk temper of this fluid fo neceffary to life, by a continual circulation. 6. Two contrary winds blowing towards the fame place, may accumulate the air there, "fo as to increase the height and the weight of the incumbent cylinder; in Like manner the direction of two winds may be fuch, 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 fame place (which may be occafioned by thunder and lightning) the height and weight of the air may be much decreased. 7. The changes in our atmofphere at high water, new and full moon, the equinoxes, &c. muft occafion alterations in all animal bodies, for all living creatures require air of a determined gravity to perform refpiration eafily; for it is by its weight that this fluid infinuates itfelf into the cavity of the breaft and lungs: by a flow circulation the fecretion of the fpirits is diminished; and by the want of the force of elafticity and gravity, the juices begin to ferment, change the union of their parts, break their canals, and difeafes follow.

Befides the above caufes, the atmosphere may be put in motion by the elastic vapours forced from the bowels of the earth by fubterraneous heats, and condenfed by whatever caufes in the atmosphere. A mixture of effluvia in different qualities in the air may, by rarefaction, fermentation, &c. produce winds and other effects like thofe refulting from the combination of fome chemical liquors; and that fuch things happen, we are affured from the nature of thunder, lightning, and meteors. From the eruption of volcanoes and earthquakes in diftant 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 fun and moon, &c. We know that there happen violent tempets in the upper region of the air, when we below enjoy a calm, and how many ridges of mountains there are on our globe which interrupt and check the propagation of the winds, fo that it is no wonder that the phoenomena we have afcribed to the action of the fun and moon, are not always conflant 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 afcends is fufficiently demonftrated by the aeroftatic globe, or air balloon, lately invented: this is a globe made of filk, or other light ftuff, made air tight with gum; which, being filled with inflammable or rarefied air, will, when let loofe, afcend, until it comes to that part of the atmosphere that is nearly as light as the air within it, where it will continue fome time.

OF

OF TIDES.

A

TIDE is that motion of the water in the feas and rivers, by

which they regularly rife and fall: the general caufe of which was discovered by Sir ISAAC NEWTON, and is deduced from the following confiderations: Daily experience fhews, that all bodies, when thrown upwards from the earth, fall down to its furface in perpendicular lines; and as lines perpendicular to the furface of any fphere tend towards its centre, the lines, along which all heavy bodies fall, muft be directed towards the earth's

centre.

As bodies appear to fall by their weight or gravity, the law, by which they defcend, is called the law of gravitation: and as a magnet or loadstone will draw fmall portions of iron or steel, and as a piece of glafs, amber, or fealing-wax, when warmed by rubbing, will draw fmall bits of paper, and other light fubftances, the law, by which fuch bodies fly to thofe 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 difcovered, by a great number of obfervations, that this law of gravitation or attraction was univerfally diffused throughout the folar fyftem; and that the regular motions, observed among the heavenly bodies, were governed by it; fo that the earth and moon attract each other, and both of them are attracted by the fun. He alfo difcovered, that the force of attraction, mutually exerted by these bodies, was leffened as the distance increased, in proportion to the fquares of those distances; that is, the power of attraction, at double the distance, was four times lefs; at triple the diftance nine times lefs; at quadruple the dif tance, fixteen times lefs, and fo on.

As the earth is attracted by the fun and moon, it follows, that all the parts of the earth will not gravitate towards its centre in the fame manner as they would do, if those parts were not affected by fuch attractions. And it is evident, that were the earth entirely free from fuch actions of the fun and moon, the ocean, being on all fides equally inclined towards its centre by the force of gravity, would continue in a perfect ftagnant ftate, without ever ebbing or flowing. But, as the cafe is other wife, the water in the ocean must needs rife higher in thofe places where the fun and moon diminish its gravity, or where they have the greatest attraction,

As

As the force of gravity must be diminished moft in those parts of the earth to which the moon is nearest, or in the zenith, because her attraction will there be moft powerful; therefore the waters, in fuch places, will rife higher, and it will in them be full fea or high-water. The parts of the earth directly under the moon, and allo thofe in the nadir, viz. fuch places as are diametrically oppofite to thofe where the moon is in the zenith, will have high-water at the fame time. For either half of the earth would gravitate, equally towards the other half, were they fuperfluous 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 directly under her being moft attracted, and confequently their gravitation towards the earth's centre moft diminished, the waters in thefe parts must be higher than in any other part of this half-earth. And in the half-carth, fartheft from the moon, the parts in the nadir being lefs attracted by her than those which are nearer, gravitate lefs towards the earth's centre, and confequently, the waters in thofe parts must be higher than they are in any other part of this half-earth.

Thofe parts of the earth where the moon appears in the horizon, or is 90 degrees diftant from the zenith and nadir, will have their loweft waters. For as the waters in the zenith and nadir rife at the fame time, the adjacent waters will press towards those places to restore the equilibrium; and, to fupply the places of these, others will move the fame way, and fo on to 90° diftant from the faid zenith and nadir: confequently the waters, in thofe places where the moon appears in the horizon, will have most liberty to defcend towards the centre; and therefore they will, in fuch places, be the loweft. Hence it plainly follows, that the ocean, if it covered the furface 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 the is in the horizon. And as the moon apparently fhifts her pofition from caft to weft in going round the earth every day, the long diameter of the spheroid, following that motion, would occafion 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 thofe tides be which happen when the is above the horizon; and the greater her meridian depreffion is, the greater will thofe tides be, which happen when he is below the horizon. The fummer day, and the winter night, tides, have a tendency to be the higheft; because the fun's fummer elevation, and his winter depreffion are greateft: this is more especially to be obferved when the moon has north declination in fummer and fouth declination in winter.

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

coming to the meridian, but about an hour after. For the moon continues to act with fome force after fhe has paffed the meridian, and by that means adds to the libratory, or waving motion, which the put the water into whilst she was on the meridian; in the fame manner as a small force applied upwards to a ball, already raised to fome height, will raise it still higher. The tides are greater than ordinary twice every month; that is, about the times of new and full moon they are called fpring tides. At thefe times the fun and moon concur to draw in the fame right line; and therefore the fea muft, under fuch joint influences, be more elevated than at other times. During the time of their conjunction, or whilft they are on the fame fide of the earth, they both confpire to raise the water in the zenith, and confequently in the nadir; and when the fun and moon are in oppofition, that is, when the earth is between them, whilst one makes high-water in the zenith and nadir, the other does the fame in the nadir and zenith. The tides are less than ordinary twice every month; that is, about the times of the first and last quarters of the moon; these are called neap-tides: because in the quarters of the moon, the fun raifes the water where the moon depreffes it, and depreffes where the moon raises the water; fo that the tides are then caused only by the difference of their actions. Hence it is neceffary to obferve, that the fpring-tides happen not exactly at the new and full moon, but generally three days after, when the attracting powers of the fun and moon have confpired for a confiderable time. In like manner the neap-tides happen about three days after the quarters, when the moon's attraction has been leffened by that of the fun for several days together.

When the moon is in her perigoum, or nearest approach to the earth, the tides rife higher than they do under the fame circumftance at other times; for, according to the laws of gravitation, the moon must attract most when he is nearest the earth. The fpring-tides are greater about the time of the equinoxes, that is, about the latter end of March and September, than at other times of the year; and the neap-tides are then lefs; because the longer diameter of the fpheroid, or the two oppofite floods, being then in the earth's equator, will defcribe a great circle of the earth; by the diurnal rotation of which, thofe floods will move fwifter, defcribing a great circle in the fame time they ufed to defcribe a lefs one, parallel to the equator; and confequently the waters being thrown more forcibly against the fhores, muft caufe them to rife higher.

The following obfervations have been made on the rife of the tides: namely, the morning tides generally differ in their rife from the evening tides. The new and full moon fpring tides rife to different heights. In winter the morning tides are higheft. In fummer the evening tides are higheft. Thus it appears, that, after a period of about fix months, the order of the highest tides are inverted; that is, the rife of the morning and evening tides will

change