parallactic motion, and becomes less and less percep tible as the velocity of the spectator diminishes, or as the distance of the seemingly moving object becomes greater. To measure the distance of the fixed stars is then equivalent to determining the amount of parallactic change in their relative positions, occasioned by the actual change of the positions from which they may be viewed by a spectator on the earth's surface.

With the sun and moon and planets, a base line equal to the earth's diameter, or about 8,000 miles, has sufficed to produce a sensible and measurable parallax; but when we extend our visual rays to a fixed star, from the extremities of this base, their directions, to our senses, are absolutely parallel, or, in other language, the parallax arising from such a base is perfectly insensible. This first effort indicates, at once, the vast distance of the objects under examination; for such is the accuracy with which minute spaces are now divided, that parallax may be detected in case the object is even 160,000 times farther distant than the length of the base line.

When the orbitual motion of the earth was first propounded by Copernicus, and it was asserted to revolve in an ellipse of nearly 600,000,000 of miles in circumference, and with a motion so swift that it passed over no less than 68,000 miles in every hour of time, the opponents of these startling doctrines exclaimed No! this is impossible; for if we are sweeping around the sun in this vast orbit, and with this amazing velocity, then ought the fixed stars to whirl round each other, as do the forest trees to the trav eler flying swiftly by them.

But the stars of heaven do not move.

Seen from

any point, and at any time, their places are ever the same,-fixed, immutable, eternal,-the bright and living witnesses of the extravagance and absurdity of this new and impossible theory. To this reasoning, which was well founded, and without sophistry, the Copernicans could only reply, that such was the enormous distance of the sphere of the fixed stars, that no perceptible change was occasioned by the revolution of the earth in its orbit. But this was mere assertion, and the opponents met the statement by this very plain exhibition of the case.-You who believe in the doctrines of Copernicus assert that the earth revolves on an axis, which, as it sweeps round the sun, remains ever parallel to itself. This axis prolonged meets the celestial sphere in a point called the north pole. Now as the earth describes an orbit of nearly 200,000,000 of miles in diameter, its axis prolonged will cut out of the sphere of the heavens a curve of equal dimensions, and the pole will appear to revolve and successively fill every point of this celestial curve in the course of the year. Now the north pole does not revolve in any such curve; it is ever fixed, and your theory is false. The Copernican could only reply that all the premises were true, but that the conclusion was false. The pole of the heavens did revolve in just such a curve as stated, but such was the distance of the sphere of the fixed stars, that this curve of 200,000,000 of miles in diameter was reduced to an invisible point!

Three hundred years have rolled away since this controversy began. The struggle has been long and arduous. The mind, baffled in one direction, has directed its energies in another-failing in one mcde of

research, it devises another, and thus struggling onward for three long centuries, it at length triumphs, The facts are developed, and the truth of the grand theory of Copernicus is vindicated and established, and the accuracy of these incredible statements is proved in the clearest manner.

As this discussion exhibits, clearly and beautifully, the progressive advances of human genius, I shall be pardoned for entering, at some length, into an examination of the various attempts which have been made to resolve the problem of the parallax of the fixed stars. Indeed, the distance of the nearest fixed star is to become the unit of measure with which we are to traverse the innumerable worlds and systems by which we are surrounded, and on the accuracy with which it shall be determined will depend the correctness of the survey which we are soon to make.

Failing entirely in obtaining any parallactic angle with a base line of 8,000 miles in length, the earth was employed to transport the observer from the first point of observation to a distance of 190,000,000 of miles, there again to erect his telescope, and to send up his second visual ray to the far distant star, in the hope of finding a parallactic angle with a base of such enormous extent.

Permit me to illustrate the nature of this investi gation. Suppose from the centre of a plane a solid granite rock, deep sunk and immoveable, rears its head far above the mists and impurities which float in the lower air. Ascending to the summit, the as tronomer hews out some rough peak into the form of a vertical shaft. To this solid shaft he bolts the me.

tallic plates which shall bear his telescope. The instrument is of a size and power commensurate with the grand objects which it is required to accomplish. Placed in a position such that its axis shall be exactly vertical, it is screw-bolted and iron-bound to the solid rock with fastenings which shall hold it from year to year, fixed and immoveable as its rocky base.

To give more perfect precision to his work, the astronomer places in the focus of his eye-piece two delicate lines made from the spider's web, of a minuteness almost mathematical, which, by crossing at right angles, form a point of the utmost precision exactly in the axis of the telescope. These are in like manner fixed immoveably in their places, and now the machinery is prepared with which the observations are to be conducted.

Suppose the observations to commence to-night.— On placing the eye to the telescope, and looking directly up to the zenith, a star enters the field of the instrument, and borne along by the diurnal motion of the heavens, advances towards the central point determined by the intersection of the spider's lines. In passing across the field of view, its minute diameter is exactly bisected by one of these delicate lines, and the exact moment, to the hundredth part of a second of time, is noted at which it passes the central point. This observation completed with all possible precision, in case no change in the apparent place of the star is produced by the revolution of the earth in its orbit, or by any other cause, on each successive night throughout the entire year the same phenomena will be repeated in the same precise order. When the hour comes round, the star will enter the field, thread

the spider's line, and reach the central poirt at the same precise instant, night after night, even for a thousand revolutions of the earth on its axis.

Such, then, is the delicate means employed in the examination of the problem of the parallax of the fixed stars; and nearly in this way did Bradley, the great English astronomer, prosecute this intricate investigation. If any change in the star's place is occasioned by the revolution of the earth in its orbit, sweeping, as it does, the spectator round the circumference of a track nearly 200,000,000 of miles in diameter, it is easy to compute, not the amount, but the direction in which these changes will be accomplished. These computations were made by the astronomer, and all things being prepared, he commenced the series of observations which were to lead to the most important results. The discovery of absolute fixity in the star would be a great negative result, and any changes, no matter of what kind or character, could not fail to be detected.

Night after night was the astronomer found at his post, and as the months rolled slowly away, he began to perceive that his star, which, for a long time, threaded the spider's line as it was in the act of passing the field of the telescope, began slowly to work off from this line, at last absolutely separating itself from it, and failing to reach the central point of the field at the precise instant first recorded. It soon became manifest that some cause or causes were operating to produce an apparent change in the place of the star, but what was the astonishment of Bradley to find that the changes in question could not be produced by parallax, for the motions detected were al