flected by, the moon. If the light of the sun is kept off from the whole body of the moon, it is a total eclipse; if from a part only it is a partial one.

An eclipse of the sun is, when the moon, being between the sun and the earth, hinders the light of the sun from coming to us. If the moon hides from us the whole body of the sun, it is a total eclipse; if not, a partial one.

Our solar system is distant from the fixt stars 20,000,000,000 semi-diameters of the earth; or, as Mr. Huygens expresses the distance, in his Cosmotheoros: the fixt stars are so remote from the earth, that if a cannon-bullet should come from one of the fixt stars with as swift a motion as it hath when it is shot out of the mouth of a cannon, it would be 700,000 years in coming to the earth.

This vast distance so much abates the attraction to those remote bodies, that its operation upon those of our system, is not at all sensible, nor would draw away or hinder the return of any of our solar comets; though some of them should go so far from the sun, as not to make the revolution about it in less than 1000 years.

It is more suitable to the wisdom, power, and greatness of God, to think that the fixt stars are all of them suns, with systems of inhabitable planets moving about them, to whose inhabitants he displays the marks of his goodness as well as to us; rather than to imagine that those very remote bodies, so little useful to us, were made only for our sake.

CHAP. IV.

OF THE EARTH, CONSIDERED AS A PLANET.

of

THE earth, by its revolution about the sun in 365 days, 5 hours, 49 minutes, makes that space 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 on 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 on 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 ro tundo," 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.

CHAP. 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 ether luminous bodies.

Though considering that the air we are in, being near 1000 times lighter than the 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 the air is 1000 times lighter than water, than a pillar of air equal in weight to a pillar of quicksilver of 30 inches high will be 35,000 feet, whereby we come to

know that the air or atmosphere is 35000 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.

That the air near the surface of the earth will mightily expand itself, when the pressure of the incumbent atmosphere 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 probable that the atmosphere may be