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CHAPTER IV.

Of the Earth, considered as a Planet.

THE earth, by its revolution about the sun 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 sun, is called ecliptic.

The annual motion of the earth about the sun, is in the order of the signs of the zodiac; that is, speaking vulgarly, from west to east.

Besides this annual revolution of the earth about the sun 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 sun 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 progressive 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 sun; and night in those parts which are in the shade, or turned from the sun.

The annual revolution of the earth in the ecliptic, is the cause of the different seasons, and of the several lengths of days and nights, in every part of the world, in the course of the year.

The reason of it, is the earth's going round its own axis in the ecliptic, but at the same 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 a half, makes that the earth, moving

round in the ecliptic, hath sometimes one of its poles, and sometimes the other, nearer the sun.

If the diameter of the sun be to the diameter of the earth as 48 to 1, as by some it is accounted; then the disk of the sun, speaking numero rotundo, is above 2000 times bigger than the disk of the earth; and the globe of the sun is above 100,000 times bigger than the globe of the earth.

The distance of the earth's orbit from the sun, is above 200,000 semi-diameters of the earth.

If a cannon-bullet should come from the sun, with the same velocity it hath when it is shot out of the mouth of a cannon, it would be 25 years in coming to the earth.

CHAPTER V.

Of the Air and Atmosphere.

We have already considered the earth as a planet, or one of the great masses of matter moving about the sun; we shall now consider it as it is made up of its several 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 encompasses the solid earth on all sides.

The height of the atmosphere, above the surface of the solid earth, is not certainly known; but that it doth reach but to a very small part of the distance betwixt the earth and the moon, may be concluded from the refraction of the rays coming from the sun, moon, and other luminous bodies.

Though considering that the air we are in, being near 1000 times lighter than water; and that the higher it is, the less it is compressed by the superior incumbent air, and so consequently being a springy body the thinner it is; and considering also that a

pillar of air of any diameter is equal in weight to a pillar of quicksilver of the same diameter of between 29 and 30 inches height; we may infer that the top of the atmosphere is not very near the surface of the solid earth.

It may be concluded, that the utmost extent of the atmosphere reaches upwards, from the surface of the solid earth that we walk on, to a good distance above us; first, if we consider that a column of air of any given diameter is equiponderant to a column of quicksilver of between 29 and 30 inches height. Now quicksilver 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 quicksilver, i. e. about 35 feet.

Secondly, if we consider that air is 1000 times lighter than water, then a pillar of air equal in weight

pillar of quicksilver of 30 inches high will be 35,000 feet; whereby we come to know that the air or atmosphere is 35,000 feet, i. e. near seven miles high.

Thirdly, if we consider that the air is a springy body, and that that, which is nearest the earth, is compressed by the weight of all the atmosphere that is above it, and rests perpendicularly upon it; we shall find that the air here, near the surface of the earth, is much denser 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 compressed by the weight of that which lies upon it; and so both of them by a third, and so on; so that, if 10,000 were piled one upon another, the under one would by the weight of all the rest be very much compressed, 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 less compressed, the third a little less than the second, and so on till it came to the uppermost, which would be in its full expansion, and not compressed at all. Just so it is in the air; the higher you go in it, the less it is compressed, and consequently the less dense it is; and so 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 assigned.

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That the air near the surface of the earth will mightily expand itself, when the pressure of the incumbent asmosphere is taken off, may be abundantly seen in the experiments made by Mr. Boyle in his pneumatic engine. In his Physico-mechanical Experiments, concerning the air, he declares* it bable that the atmosphere may be several hundred miles high; which is easy to be admitted, when we consider what he proves in another part of the same treatise, viz. that the air here about the surface of the earth, when the pressure is taken from it, will dilate itself about 152 times.

The atmosphere is the scene of the meteors; and therein is collected the matter of rain, hail, snow, thunder, and lightning; and a great many other things observable in the air.

CHAPTER VI.

Of Meteors in general.

BESIDES the springy particles of pure air, the atmosphere is made up of several steams or minute particles of several sorts, rising from the earth and the waters, and floating in the air, which is a fluid body, and though much finer and thinner, may be considered in respect of its fluidity to be like water, and so capable, like other liquors, of having heterogeneous particles floating in it.

*New Experiments Physico-mechanical, touching the spring of the air, and its effects; (made for the most part in a new pneumatical engine) written.... by the honourable Robert Boyle, Esq. experiment xxxvi. p. 155. Oxford, 1662, in 4to.

The most remarkable of them are, first, the particles of water raised into the atmosphere, chiefly by the heat of the sun, out of the sea and other waters, and the surface of the earth; from whence it falls in dew, rain, hail, and snow.

Out of the vapours rising from moisture the clouds are principally made.

Clouds do not consist wholly of watery parts; for, besides the aqueous vapours that are raised into the air, there are also sulphureous and saline particles that are raised up, and in the clouds mixed with the aqueous particles, the effects whereof are sometimes very sensible; as particularly in lightning and thunder, when the sulphureous and nitrous particles firing break out with that violence of light and noise, which is observable in thunder, and very much resembles gunpowder.

That there are nitrous particles raised into the air is evident from the nourishment which rain gives to vegetables more than any other water; and also by the collection of nitre or salt-petre in heaps of earth, out of which it has been extracted, if they be exposed to the air, so as to be kept from rain; not to mention other efforts, wherein the nitrous spirit in the air shows itself.

Clouds are the greatest and most considerable of all the meteors, as furnishing matter and plenty to the earth. They consist of very small drops of water, and are elevated a good distance above the surface of the earth; for a cloud is nothing but a mist flying high in the air, as a mist is nothing but a cloud here below.

How vapours are raised into the air in invisible steams by the heat of the sun out of the sea, and moist parts of the earth, is easily understood; and there is a visible instance of it in ordinary distillations. But how these steams are collected into drops, which bring back the water again, is not so easy to determine.

To those that will carefully observe, perhaps it will appear probable, that it is by that which the chy

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