even appeared larger than Jupiter. Kepler wrote a dissertation upon it.

Changes have also taken place in the lustre of the permanent stars; ẞ Aquila is now considerably less bright than Y. A small star near & ursa majoris is now probably more bright than formerly, from the circumstance of its being named Alcor, an Arabic word which imports sharp-sightedness in the person who could see it. It is now very visible.

26. Several stars also change their lustre periodically; o Ceti, in a period of 333 days, varies from the 2nd to the 6th magnitude. The most striking of all is Algol or ẞ Persei. Mr. Goodricke has with great care determined its periodical variations. It is, when brightest, of the 2nd, and, when least, of the 4th magnitude; its period is only 2a 21h: it changes from the second to the fourth magnitude in 34 hours, and back again in the same time, and so remains for the rest of the 2a 21h. These singular appearances may be explained, by supposing the fixed star to be a body revolving on an axis, having parts of its surface not luminous. postaje en tant foodies

27. The number of nebulæ is very considerable. Dr. Herschel has discovered above 2000: before his time only 103 were known. But far the greater part of these 2000 can be only seen with telescopes equal to his own. The vast quantity of light obtained by his large speculums, renders his telescopes very useful for discoveries among the fixed stars, for which light is the principal thing to be desired. He has given an account of several phenomena, which he calls nebulous stars, stars surrounded with a faint luminous atmosphere. He describes one observed Nov. 13, 1790. "A most singular phenomenon: a "star of the 8th magnitude, with a faint luminous atmosphere, "of a circular form, and of about 3' diameter; the star is perfectly in the centre, and the atmosphere is so diluted, faint, " and equal throughout, that there can be no surmise of its con

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sisting of stars; nor can there be a doubt of the evident con"nection between the atmosphere and the star. Another star, "not much less in brightness, and in the same field with the "above, was perfectly free from any such appearance." Phil. Trans. 1791.

Dr. Herschel has, with unwearied attention, exerted himself in examining and noting every thing remarkable in every part of the visible celestial surface, by a regular review, so that little can escape him. In consequence of his numerous discoveries, many very ingenious and magnificent ideas have occurred to him respecting the fixed stars and nebulæ.

28. Having given a short statement of the simple appearances of the bodies placed on the concave surface of the heavens, which are such, that they must strongly excite our curiosity; we may now leave the subject, and resume it after having stated the knowledge that observations and deductions from thence afford us, respecting the magnitudes, distances, and motions of the sun, moon, and planets. Then returning again to the consideration of the fixed stars, and assigning them their proper places in the universe, we shall discover what must fill our minds with astonishment and awe, and must raise in us the greatest admiration of the Almighty Creator. That which has hitherto been stated, regards only what a spectator fixed to one spot might discover. It is only by a change of place, or by comparing the observations made at places distant from each other, that we can readily arrive at a knowledge of the real distances and real motions of the celestial bodies. An isolated observer, however he might be gratified by the spectacle of the heavens on a fine evening, would be able to discover little of what, when the true circumstances are known, add so much to the wonderful variety we observe in terrestrial matters, of the Creator's power. He would only barely discover that the sun, moon, and planets were. at different distances from the earth.

He would also be able to form hypotheses to explain their motions, but few of those would he be enabled to submit to the test of experience. Previously to this it would be necessary to investigate the figure and dimensions of the earth upon which he lives. This knowledge is obtained from the phenomena which arise from a change of place.

CHAPTER III.

PHENOMENA DEPENDING ON A CHANGE OF PLACE, AND ON THE FIGURE OF THE EARTH.

29. A SPECTATOR, without changing his situation on the earth, would soon discover that the celestial bodies are not all placed on the concave surface at fixed distances from him; for he would remark that the sun, moon, and planets varied their apparent magnitudes or diameters, which must arise either from changes of distance, or changes in the actual magnitudes of the bodies. The former solution is so much simpler than the latter, that no one could hesitate in adopting it, even if not confirmed by other circumstances. Likewise that the heavenly bodies are not placed at equal distances from him. It was remarked that the apparent paths of the sun and moon intersected each other. When they appear to meet at these intersections, the moon is observed to obscure or eclipse the sun, consequently the moon must be nearer than the sun. But to proceed in the investigation of these distances, it will, as was observed, be necessary to become acquainted with the form of the earth on which we live.

30. A spectator placed on the sea, or on a plain, where his view is unobstructed, at first considers the surface as a plane coinciding with his horizon, and extended to the concave surface of the celestial sphere. But it is immediately suggested to him, that the surface of the earth is not flat or coincident with his horizon, for on the sea he perceives the tops of the masts to dis

appear last, and on the plain he observes the tops of distant objects, when the bottoms are invisible. This cannot be explained otherwise than by a curvature on the earth's surface. The voyages of modern navigators have put this matter in the clearest light; for, by continued sailing to the eastward or westward, they have arrived again at the port from which they set out. This has been done in different courses on the surface, so that thereby traversing the earth, they have ascertained its surface to be a curved surface returning into itself. Eclipses of the moon serve to point out that the figure of the earth must be nearly spherical, for the boundary of the earth's shadow seen on the moon always appears circular, which could not always be the case, unless the earth were nearly a sphere.

31. The magnitude of the earth is next to be considered; previously to which it is necessary to remark, that however distant two places on the earth's surface are, the angular distances of the same stars visible in each place are precisely the same; from whence it follows, that the distances of the fixed stars are so great, that each inhabitant of the earth, in respect to them, considers himself in the centre of the same imaginary sphere; or that all lines drawn from the surface of the earth to any star, may be considered as parallel at the surface of the earth; for the inclination of the lines drawn from any two places towards the same star, is less than can be measured, and therefore for all purposes they must be considered as parallel.

32. Every spectator also observes the same celestial pole and equator, that is, situate the same with respect to the fixed stars; but the situation of the celestial circles with respect to the horizon will be different. The meridian altitudes of the celestial objects will be different in different places, and the altitude of the north celestial pole will be increased or diminished, according as an observer travels north or south. Actual admeasurement shews, that the space gone over in a direction