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(call it which you please); and an oblique or fide-long impulfe or motion.
Thefe two motions or tendencies, the one always endeavouring to carry them in a straight line from the circle they move in, and the other endeavouring to draw them in a straight line to the fun, makes that curve line they revolve in.
The motion of the comets about the fun is in a very long flender oval : whereof one of the focuses is the centre of the fun, and the other very much beyond the sphere of Saturn.
The moon moves about the earth, as the earth doth about the fun. So that it hath the centre of its motion in the earth; as the earth hath the centre of its revolution in the fun, about which it moves.
The moon makes its fynodical motion about the earth, in 29 days, 12 hours, and about 44 minutes.
It is full moon, when, the earth being between the fun and the moon, we fee all the enlightened part of the moon; new moon, when, the moon being between us and the fun, its enlightened part is turned from us; and half moon, when the moon being in the quadratures, as the aftronomers call it, we fee but half the enlightened part.
An eclipse of the moon is, when the earth, being between the fun and the moon, hinders the light of the fun from falling upon, and being reflected by, the moon. If the light of the fun is kept off from the whole body of the moon, it is a total eclipfe; if from a part only, it is a partial one.
An eclipfe of the fun is, when the moon, being between the fun and the earth, hinders the light of the fun from coming to us. If the moon hides from us the whole body of the fun, it is a total eclipse; if not, a partial one.
Our folar fyftem is diftant from the fixt ftars 20,000,000,000 femi-diameters of the earth; or, as Mr. Huygens expreffes the distance, in his Cofmotheoros*: the fixt stars are so remote from the earth, that,
* Chriftiani Huygenii ΚΟΣΜΟΘΕΩΡΟΣ, five de terris caleftibus earumque ornatu, conjecture, &c. p. m. 137.
if a cannon-bullet fhould come from one of the fixt stars with as fwift a motion as it hath when it is fhot out of the mouth of a cannon, it would be 700,000 years in coming to the earth.
This vaft diftance fo much abates the attraction to those remote bodies, that its operation upon those of our system is not at all fenfible, nor would draw away or hinder the return of any of our folar comets; though some of them fhould go so far from the fun, as not to make the revolution about it in lefs than 1000 years.
It is more fuitable to the wifdom, power, and greatnefs of God, to think that the fixt stars are all of them funs, with systems of inhabitable planets moving about them, to whofe inhabitants he displays the marks of his goodness as well as to us; rather than to imagine that those very remote bodics, fo little useful to us, were made only for our fake.
HE earth, by its revolution about the fun in 365 days, 5 hours, 49 minutes, makes that space of time we call a year.
The line, which the centre of the earth describes in its annual revolution about the fun, is called ecliptic. The annual motion of the earth about the fun, is in the order of the figns of the zodiac; that is, speaking vulgarly, from west to east.
Befides this annual revolution of the earth about the fun in the ecliptic, the earth turns round upon its own axis in 24 hours.
The turning of the earth upon its own axis every 24 hours, whilst it moves round the fun in a year, we may conceive by the running of a bowl on a bowling-green; in which not only the centre of the bowl hath a progreffive motion on the green; but the bowl in its going forward, from one part of the green to another, turns round about its own axis.
The turning of the earth on its own axis, makes the difference of day and night; it being day in those parts of the earth which are turned towards the fun; and night in those parts which are in the fhade, or turned from the fun.
The annual revolution of the earth in the ecliptic, is the cause of the different feafons, and of the several lengths of days and nights, in every part of the world, in the course of the year.
The reafon of it, is the earth's going round its own axis in the ecliptic, but at the fame time keeping every where its axis equally inclined to the plane of the ecliptic, and parallel to itself. For the plane of the ecliptic inclining to the plane of the equator, 23 degrees and an half, makes that the earth, moving round in the ecliptic, hath fometimes one of its poles, and sometimes the other, nearer the fun.
If the diameter of the fun be to the diameter of the earth, as 48 to 1, as by fome it is accounted; then the difk of the fun, fpeaking " numero rotundo," is above 2000 times bigger than the difk of the earth; and the globe of the fun is above 100,000 times bigger than the globe of the earth.
The diftance of the earth's orbit from the fun, is above 200,000 femi-diameters of the earth.
If a cannon-bullet fhould come from the fun, with the fame velocity it hath when it is fhot out of the mouth of a cannon, it would be 25 years in coming to the earth.
Of the Air and Atmosphere.
E have already confidered the earth as a planet, or one of the great maffes of matter moving about the fun; we fhall now confider it as it is made up of its feveral parts, abstractedly from its diurnal and annual motions,
The exterior part of this our habitable world is the air or atmosphere; a light, thin fluid, or springy body, that encompaffes the folid earth on all fides.
The height of the atmofphere, above the furface of the folid earth, is not certainly known; but that it doth reach but to a very fmall part of the diftance betwixt the earth and the moon, may be concluded from the refraction of the rays coming from the fun, moon, and other luminous bodies.
Though confidering that the air we are in, being near 1000 times lighter than water; and that the higher it is, the lefs it is compreffed by the fuperior incumbent air, and fo confequently being a fpringy. body the thinner it is; and confidering alfo that a pillar of air of any diameter is equal in weight to a pillar of quickfilver of the fame diameter of between 29 and 30 inches height; we may infer that the top of the atmosphere is not very near the furface of the folid earth.
It may be concluded, that the utmost extent of the atmosphere reaches upwards, from the furface of the folid earth that we walk on, to a good distance above us; firft, if we confider that a column of air of any given diameter is equiponderant to a column of quickfilver of between 29 and 30 inches height. Now quickfilver being near 14 times heavier than water, if air was as heavy as water, the atmosphere would be about 14 times higher than the column of quickfilver, i. e. about 35 feet.
Secondly, if we confider that air is 1000 times lighter than water, then a pillar of air equal in weight to a pillar of quickfilver of 30 inches high will be 35000 feet; whereby we come to know that the air or atmosphere is 35000 feet, i. e. near feven miles high.
Thirdly, if we confider that the air is a fpringy body, and that that, which is neareft the earth, is compreffed by the weight of all the atmosphere that is above it, and refts perpendicularly upon it; we fhall find that the air here, near the furface of the earth, is much denfer and thicker than it is in the upper parts. For example, if upon a fleece of wool you lay another; the under one will be a little compreffed by the weight of that which
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lies upon it; and fo both of them by a third, and fo on ; fo that, if 10000 were piled one upon another, the under one would by the weight of all the reft be very much compreffed, and all the parts of it be brought abundantly closer together, than when there was no other upon it; and the next to that a little lefs compreffed, the third a little lefs than the fecond, and fo on till it came to the uppermoft, which would be in its full expanfion, and not compreffed at all. Juft fo it is in the air; the higher you go in it, the lefs it is comprefied, and confequently the lefs dense it is; and fo the upper part being exceedingly thinner than the lower part, which we breathe in (which is that that is 1000 times lighter than water); the top of the atmosphere is probably much higher than the distance above affigned.
That the air near the furface of the earth will mightily expand itself, when the preffure of the incumbent atmosphere is taken off, may be abundantly feen in the experiments made by Mr. Boyle in his pneumatic engine. In his "Phyfico-mechanical Experiments," concerning the air, he declares it probable that the atmosphere may be feveral hundred miles high; which is eafy to be admitted, when we confider what he proves in another part of the fame treatise, viz. that the air here about the furface of the earth, when the preffure is taken from it, will dilate itself about 152 times.
The atmosphere is the fcene of the meteors; and therein is collected the matter of rain, hail, fnow, thunder, and lightning; and a great many other things obfervable in the air.
New Experiments Phyfico-mechanical, touching the fpring of the air, and its effects; (made for the most part in a new pneumatical engine) written.... by the honourable ROBERT BOYLE, Efq; experiment xxxvi. p. 155. Oxford, 1662, in 4to.