all that part of the earth which was open to his view; so, in the same manner, no sooner was an idea formed of the vast extent and greatness of the universe, with respect to this earth, than mankind began to conceive it would be more rational that the earth should move, than the whole fabric of the heavens.

By another familiar instance it will be easy to shew the young pupil, that as the eye does not perceive its own motion, it always judges from appearances. Let a person go into a common windmill, and desire the miller to turn the mill round, while he is setting within, with his eyes fixed on the upright post in the centre thereof; this post, though at rest, will appear to him to turn round with considerable velocity, the real motion of the mill being the cause of the apparent motion of the swivel-post. Seafaring people are furnished with various instances to illustrate this subject; those who are busy in the hold of a ship at anchor, cannot by any perception determine whether the ship has swung round or not by the turn of the tide. When a ship first gets under-way with a light breeze, she may be going at a good rate, before those who are between decks can perceive it. Having thus obviated the objections which arise from the testimony of the senses, we may now proceed to consider the arguments which tend more directly to prove the motion of the earth.

All the celestial motions will, on this supposition, be incomparably more simple and moderate.

This opinion is much more agreeable to our no tions of final causes, and our knowledge of the economy of nature; for if the earth be at rest, and the stars, &c. move round it once in 24 hours, their yelocity must be immense; and it is certainly more agreeable to reason, that one single body, and that one of the smallest, should revolve on its own axis in 24 hours, than that the whole universe should be carried round it in the same time, with inconceivable velocity.

The rotation of the earth round its axis is analogous to what is observed in the sun, and most of the planets; it being highly probable that the earth, which is itself one of the planets, should have the same motion as they have, for producing the same effect; and it would be as absurd in us to contend for the motion of the whole heavens round us in 24 hours, rather than allow a diurnal motion to our globe, as it would be for the inhabitants of Jupiter to insist that our globe and the whole heavens must revolve round them in 10 hours, that all its parts might successively enjoy the light, rather than grant a diurnal motion to their habitation.

All the phenomena relative to this subject, are as easily solved on the supposition of the earth's motion, as on the contrary hypothesis.

Besides the foregoing considerations, there are several arguments to be deduced from the higher parts of astronomy, which demonstrably prove the diurnal motion of the earth.

Before we enter into a further explanation of phe

nomena, it will be necessary to define some of the principal circles of the globe. The reader will comprehend more fully these definitions, and attain more accurate ideas of these circles, by placing, while he is reading them, a terrestrial globe or armillary sphere before him. It may, however, be however, be necessary to premise, that we are at liberty to suppose as many circles as we please to be described on the earth; and the plane of any of these to be continued from the earth, until it marks a corresponding circle in the concave sphere of the heavens.

Among these circles, the horizon is the most fre quently named. Properly speaking, there are two circles by this name, but distinguished from each other by added epithets; the one being called the sensible, the other the rational horizon.

In general terms, the horizon may be defined to be an imaginary circle, that separates the visible from the invisible part of the heavens.

If a spectator supposes the floor or plane on which he stands, to be extended every way, till it reach the starry heavens, this plane is his sensible horizon.

The rational horizon is a circle, whose plane is parallel to the former, but passing through the cen tre of the earth.

The rational horizon divides the concave sphere of the heavens into two equal parts, or hemispheres ; the objects that are in the upper hemisphere will be visible; such as are in the lower hemisphere will be invisible to the spectator.

Though the globe of the earth appears so large.

to those who inhabit it, yet it is so minute a speck when compared to the immense sphere of the hea vens, that at that distance the planets of the rational and sensible horizons coincide; or, in other words, the distance between them in the sphere of the heavens is too small for admeasurement.

To illustrate this, let ABCD, plate 3, fig. 1, represent the earth; zhno the sphere of the starry heaven. If an inhabitant of the earth stand upon the point A, his sensible horizon is se, his rational one ho; the distance between the planes of these two horizons is AF, the semi-diameter of the earth, which is measured in a great circle upon the sphere of the heaven, by the angle eFo, or the arc eo; this arc, in so small a circle, zhno, would amount to several degrees, and consequently the difference between the sensible and rational horizon would be great enough to be measured by observation. If we represent the sphere of the heaven by a larger circle, the semi-diameter of the earth AF, measured in this circle, will amount to fewer degrees; for the are EO is less than the arc ee; and the larger the sphere of the heaven is, in proportion to the globe of the earth, the less sensible is the difference between the two horizons. Now, as the sphere of the earth is but as a point when compared to the starry heaven, the difference between the sensible and rational horizon will be insensible.

From what has been said, it appears that the only distinction between the sensible and rational horizon,

arises from the distance of the object we are looking at.

The sensible horizon is an imaginary circle, which terminates our view, when the objects we are looking at are upon the earth's surface.

The rational horizon is an imaginary circle, which terminates our view, when the objects we are looking at are as remote as the heavenly bodies.

As the rational horizon divides the apparent celestial sphere into two equal hemispheres, and serves as a boundary, from which to measure the elevation or depression of celestial objects; those in the upper, or visible hemisphere, are said to be high, or elevated above the horizon; and those in the other hemisphere are called low, or below the horizon.

The earth being a spherical body, the horizon, or limits of our view, must change as we change our place; and, therefore, every place upon the earth has a different horizon. Thus, if a man lives at a, plate 3, fig. 2, his horizon is GC; if he lives at b, his horizon is HD; if at c, it is AE. From hence we obtain another proof of the sphericity of the earth ; for if it were flat, all the inhabitants thereof would have the same horizon.

The point in the heavens, which is directly over the head of a spectator, is called the zenith.

That point which is directly under his feet, is called the nadir.

If a man lives at a plate 3, fig 2, his zenith is A, his nadir E; if he lives at b, his zenith is B, his nadir F. Consequently, the zenith and horizon