Clusters of stars, (Plate X. Fig. 6) differ from groups, in their beautiful and artificial arrangement; regarded by Dr. Herschel among the most magnificent objects in the heavens. Nebula, or cloudy stars, are bright spots in the heavens, (Plate XI. Fig. 1, 2, and 3.) Some of them are found to be clusters of telescopic stars. tween the two stars Huygens in 1656. The most noted nebula is bein the sword of Orion, discovered by It contains seven stars, and in another part, a bright spot upon a dark ground, seeming to be an opening into a brighter and more distant region, (Plate XI. Fig. 1.) Nebula were discovered by Dr. Halley and others. A catalogue of 103 nebulæ, discovered by Messier and Meihain, is inserted in the Connoisance de tems, for 1783 and 1784. "But to Dr. Herschel," says Enfield, "are we indebted for catalogues of 2000 nebulæ and clusters of stars, which he himself has discovered." Dr. Brewster says 2500. We cannot contemplate the fixed stars without repeating the sentiment expressed in the introduction; without admiration and astonishment! How inconceivably Great and Wise and Good must be the AUTHOR and GOVERNOR of all these! We behold, not one world only, but a system of worlds, regulated and kept in motion by the sun; not one sun and one system only, but millions of suns and of systems, multiplied without end, never conflicting, always revolving in harmonious order! CHAPTER XIII. REFRACTION. Refraction of light is the incurvation of a ray from its rectilinear course, in passing from a medium into one of different density. On entering a denser medium, a ray coming obliquely is turned towards the perpendicular drawn to its surface. But it is turned from the perpendicular, to the surface of a rarer medium, when passing from one more dense. An object always appears in the direction, in which a ray of light from it meets the eye of the observer; though before it may have passed in different directions. Let A B C D, (Plate VIII, Fig. 7.) be a vessel filled with water up to the line E F, and the remainder filled with spirit, or other transparent fluid, less dense than water. Let a ray of light be reflected from an object a at the bottom in the direction a I. This ray in passing from the water into the fluid less dense would be refracted from the perpendicular to the water's surface in a new direction, as to H, where the object at the bottom, without further refraction, would appear at b. But on leaving the rarer medium at the surface, A B, it is again refracted, and again passes in a new direction, as to G. So that an eye at G must see the object a at c. A straight rod appears crooked, when partially immersed in water, viewed obliquely to its surface. Put a piece of money into a bowl, (Plate VIII, Fig. 5.) and retire till the money is just hidden by the edge of the bowl; let an attendant pour water into the bowl, the money will rise into view.When the eye is perpendicular to the surface of the medium, there is no refraction. In wading a river, the water, where you are, appears of its proper depth; but at a little distance forward it seems more shallow, than on trial it will be found. It has been said, no doubt with truth, that this circumstance has often been the cause of drowning. The light of heavenly bodies is refracted by the atmosphere of the earth. (Plate V, Fig.5.) This refraction, greatest at the horizon, decreases towards the zenith, where it becomes nothing. When a medium is throughout equally dense, the refraction is at the surface. But the atmosphere increases in density from its utmost height to the surface of the earth. A ray of light must therefore be more and more bent, and pass in a curvilinear course through the atmosphere. The refraction brings up a heavenly body, before it arrives at the horizon. Let A B C (Plate VIII, Fig. 9.) be a part of the earth's atmosphere, B F, the sensible horizon to a spectator at B, D a place of the sun below the horizon. Suppose a ray of light, passing in the direction D F, should strike the atmosphere at F. This would be refracted by the increased density of the air all the way from F to the surface of the earth, as to B ; and would present the sun in the line of its last direction, B E. The sun would, therefore, appear in the horizon, before it arrived at that circle. Cold increases the density of the air, and of course the refracting power. In general, the higher the latitude the greater the refraction. Mr. Ferguson tells us, that the sun arose to some Hollanders, who wintered in Nova Zembla, in 1596, seventeen days sooner, than by calculation it would have been above the horizon. re As the horizontal refraction in latitude 43°, is about 33', the sun's mean diameter about 32,' the sun must be visible, when more than his whole breadth below the horizon. The horizon in such latitude passing these 33' obliquely, and quiring about three minutes of time, the sun is about three minutes in the morning and three at night, longer above the horizon on account of the refraction, increasing the length of the day about six minutes. The refraction of the atmosphere is sometimes the cause of a curious phenomenon, the sun and moon both visible, when the moon is eclipsed by the earth's shadow. Mr. Phillips mentions an instance of this kind, observed at Paris in 1750. The progressive motion of light has been shown to be another cause, why a heavenly body does not appear in its true place. But this does not alter the length of the day; for as the sun appears below its true place in the morning, it appears above it at night, leaving the length of the day unaffected. The disk of the sun or moon appears elliptical, when in or near the horizon. The lower limb being more refracted than the upper, not only by the atmosphere itself, but often by the floating vapour, the outline of the disk must be changed from a circle to an elliptical form. The table of refraction, here inserted, is that of Dr. Bradley, extracted from Enfield. It agrees very nearly with Mr. Bowditch's table of refraction as set to degrees. MEAN ASTRONOMICAL REFRACTIONS IN ALTITUDE. App. Refrac- [app. Refrac-app. Refrac-japp. Refrac-app. Rfrac-app., Refrac alt. alt. tion. alt. tion. alt. tion. alt. tion. alt tion tion 033 011 4 47 232 14351 21 43 2 7 36 1 18 50 45 26 2 371 16 52 1 56381 1355 1 51|39|1 1058 17 28 1 4740 1 860 4291 42 41 1 562 365 26 The twilight is the result of refraction. The atmosphere of the earth extends about 45 miles above the surface; or at that height is sufficiently dense to refract the rays of the sun. Hence it is found, that when the sun is about 18° below the horizon, the morning twilight will begin, and the evening twilight end. It is said however, that the evening twilight is longer than that of the morning. This may be owing to the elevation of the atmosphere by the heat of the day, and also to the vapour exhaled by rarefaction. The continuance of twilight, increasing with the distance from the equator, must be very long in high latitudes. At the poles the sun is never more than about 23° 28 below the horizon. To a polar inhabitant, if any, it must be more than 50 days after the sun sets, before it will be 18° below the horizon; and the same time, on its return, after it approaches within 18°, before it will be above the horizon. Here we must be led to contemplate with admiration the immense benefit of the atmosphere. Not only by the chemical operations of air, does it cause our blood to flow, and diffuse warmth through our bodies; but, by its reflecting and refracting powers, it gives beauty to our day, and enlarges its borders, even into the regions of night. Astronomers generally concur with Dr. Keill; "That it is entirely owing to the atmosphere, that the heavens appear bright in the day time. For without it, only that part of the heavens would be luminous in which the sun is placed, and, if we could live without air, and should turn our backs to the sun, the whole heavens would appear as dark as in the night. In this case also, we should have no twilight, but a sudden transition from the brightest sunshine to dark night, immediately upon the setting of the sun, which would be extremely inconvenient, if not fatal to the eyes of mortals." CHAPTER XV. ZODIACAL LIGHT. Zodiacal light seems to have been seen by Descartes in 1659, yet it attracted no general notice, till observed by Cassini in the year 1693, when it received its present name. This light, less brilliant than the milky way, appears at certain seasons of the year, in the morning before the rising of |