actual distances from us. Telescopes enable us to examine more exactly their appearances, and serve to exhibit many most interesting phenomena, but do not directly lead us further.

The first step of importance is a knowledge of the form and magnitude of the earth. The fixed stars appear in the same relative situations, at the same angular distances from each other, and from the visible celestial pole, in whatever part of the earth we are. The most exquisite instruments point out no alteration. The conclusion drawn from this is, that the fixed stars are at distances so great, that lines directed from all places on the surface of the earth towards the same fixed star, or towards the visible celestial pole, must be considered as parallel. Combining this with what has been observed in so many places, that the variation of altitude of the celestial pole is proportional to the space gone over in a direction north or south, and that for a change of altitude of one degree, the space is about 69 miles, it is easily proved that the earth is nearly a sphere of about 8000 miles in diameter.

This is an important step-We thus ascertain that a space of 8000 miles is as nothing compared with the distances of the fixed stars.

It also follows that the altitude of the celestial pole is equal to the latitude of the place. This conclusion enables us to solve the problems arising from the situation of the celestial circles in different places, and to explain the variety

of seasons over the whole earth, independent of the know

ledge of the true system.

Having ascertained the form and magnitude of the Earth, the next step is to investigate the magnitudes of the sun and planets, or at least to show, that some of them greatly exceed the earth in magnitude, and also to show the vast distances of them compared with the diameter of the earth. It is important that this should be done previously to demonstrating the true system.

Certain observations, made with micrometers, at two places considerably distant from each other, but nearly under the same meridian, serve for this purpose. The student will readily apprehend this method; he will see, that by it we are enabled to ascertain the angle, the disc of the earth would be seen under, could we remove ourselves to a planet to make the observation. This angle can be ascertained with as great precision, as we can measure the apparent diameter of a planet seen from the earth. If, with respect to some of the planets, the angle which the earth's disc subtends be so small, that it is within the limit of the errors of observation, yet we obtain a limit of the magnitude of the earth compared with the magnitude of the planet. Thus the earth seen from Jupiter subtends an angle of four seconds, when Jupiter seen from the earth subtends an angle of forty seconds. Now if it be contended that neither of these angles can be ascertained to a second or two, it will make no difference as to the purpose, for which this mode

of ascertaining the relative magnitudes of the earth and Jupiter is introduced. It will sufficiently show that the

magnitude of Jupiter greatly exceeds that of the earth, and also will show, that the distance of Jupiter is many thousand times greater than the diameter of the earth.

The spots upon the sun, and appearances in several of the planets, show that they are spherical bodies, having a motion of rotation on their axes. All this, being quite independent of any hypothesis as to the arrangement of these bodies, assists much in the arguments by which the rotation of the earth on its axis, and its annual motion round the sun in an orbit nearly circular, may be proved.

The different motions of the planets on the concave surface which appear so irregular, are easily explained by their moving in orbits nearly circular about the sun.

By following an arrangement of this kind, any student may without difficulty satisfy himself of the truth of the Copernican system. He will find this manner of treating the subject pursued in the first seven chapters of this work. At the end of the seventh chapter is a short account of the Ptolemaic System, now no longer interesting, except on account of the ingenuity exhibited in accommodating it to the different phenomena.

After the true system has been explained, the subsequent arrangement in a treatise on astronomy seems of little

consequence.

In this work, after the motions of the primary planets

are explained in a general manner, the motions of the moon and secondary planets and several other circumstances connected therewith are briefly noticed. This is followed by some considerations respecting the solar system and fixed

stars.

A short account of instruments and observations, by which the places and motions of the celestial bodies are exactly ascertained, is followed by a more exact statement of the planetary motions, and by an account of Kepler's discoveries; also by a more particular account of the motions of the moon, of the satellites and of comets.

Several of the phenomena, which arise from, or are pointed out by, the motions and bodies of the solar system, are next considered. Such are the eclipses of the Sun and Moon, the transits of Venus and Mercury over the Sun's disc, the velocity and aberration of light, and the equation of time.

The application of astronomy to navigation and geography is also introduced, and the importance of the former has occasioned a rather long detail.

The chapter on the discoveries in physical astronomy contains little more than an historical account. It had been at first intended that it should contain the elementary parts of physical astronomy, as far as respected Kepler's discoveries. Physical and plane astronomy are now so connected that it is difficult to treat of them separately.

Facts in the history of astronomy have been only occa

sionally introduced. The student, who has made himself so well acquainted with astronomy as to find its history interesting, will easily procure for himself, from a variety of authors, all the information he can desire.

Among the various advantages derived from the science of astronomy, there is one eminently deserving of notice. We see the most complex appearances and most intricate apparent motions admitting of the simplest explanations.

How intricate and various are the apparent motions which depend only on a primary motion of projection and the simple law of gravity! This may assist us in other departments of natural science, and may encourage us to expect that the most difficult phenomena may at last be found to arise from the most simple laws.

The aberration of light furnishes a remarkable illustration.

Light moves about 200,000 miles in a second; had it moved only 50 miles in a second, it is probable astronomy would not now have existed as a science. The motions of the stars and planets would have appeared inextricable confusion. The face of the heavens would have been continually changing, and could not have been divided into constellations. Stars which at one time would be seen close together, at another would appear many degrees asunder. All this would be occasioned by the simple change of the velocity of light, and, as is easily understood, would arise