eclipsed when she is at different distances from the earth. Charles. And according as the moon is nearer to, or farther from the earth, the eclipse will be of a greater or less duration, for the shadow being conical, becomes less and less, as the distance from the body by which it is cast is greater. Tutor. It is by knowing exactly at what dis tance the moon is from the earth, and of course the width of the earth's shadow at that distance, that all eclipses are calculated with the greatest accuracy, for many years before they happen. Now, it is found that in all eclipses, the shadow of the earth is conical, which is a demonstration, that the body by which it is projected is of a spherical form, for no other sort of figure would, in all positions, cast a conical shadow. This is mentioned as another proof, that the earth is a spherical body. James. It seems to me to prove another thing, viz. that the sun must be a larger body than the earth. Tutor. Your conclusion is just, for if the two bodies were equal to one another (Plate VII. Fig. 13.) the shadow would be cylindrical; and if the earth were the larger body, (Plate vii. Fig. 14.) its shadow would be of the figure of a cone, which had lost its vertex, and the farther it were extended the larger would it become. In either case the shadow would run out to an infi nite space, and accordingly must sometimes involve in it the other planets, and eclipse them, which is contrary to fact. Therefore, since the earth is neither larger than, nor equal to the sun, it must be the lesser body. We will now proceed to the eclipses of the sun. Charles. How are these occasioned? Tutor. An eclipse of the sun happens when the moon м, passing between the sun's and the earth T (Plate VII. Fig. 15.) intercepts the sun's light. James. The sun then can be eclipsed only at the new moon. Tutor. Certainly; for it is only when the moon is in conjunction that it can pass directly between the sun and earth. Charles. It is only when the moon at her conjunction, is near one of its nodes, that there can be an eclipse of the sun? Tutor. An eclipse of the sun depends upon this eircumstance for unless the moon is in, or near one of its nodes, she cannot appear in the same plane with the sun, or seem to pass over his disk. In every other part of the orbit she will appear above or below the sun. If the moon be in one of the nodes, she will, in most cases, cover the whole disk of the sun, and produce a total eclipse; if she be any where within about 16 degrees of a node, a partial eclipse will be produced. The sun's diameter is supposed to be divided into 12 equal parts, called digits, and in every partial eclipse, as many of these parts of the sun's diameter as the moon covers, so many digits are said to be eclipsed. James. I have heard of annular eclipses, what are they, sir? Tutor. When a ring of light appears round the edge of the moon during an eclipse of the sun, it is said to be annular, from the Latin word annulus, a ring: these kind of eclipses are occasioned by the moon being at her greatest distance from the earth at the time of an eclipse, because in that situation the vertex or tip of the cone of the moon's shadow does not reach the surface of the earth. Charles. How long can an eclipse of the sun last? Tutor. A total eclipse of the sun is a very curious and uncommon spectacle and total darkness cannot last more than three or four minutes. Of one that was observed in Portugal, 150 years ago, it is said that the darkness was greater than that of the night:-that stars of the first magnitude made their appearance ;and that the birds were so terrified that they fell to the ground. James. Was this visible only at Portugal? Tutor. It must have been seen at other places, though we have no account of it. The moon, however, being a body much smaller than the earth, and having also a conical shadow, can with that shadow only cover a small part of the VOL. I.-S earth; whereas an eclipse of the moon may be seen by all those that are on the hemisphere which is turned towards it. (See Plate VII. Fig. 15 and 12.) You will also observe, that an eclipse of the sun may be total to the inhabitants near the middle of the earth's disk, and annular to those in places near the edges of the disk, for in the former case the moon's shadow will reach the earth, and in the latter, on account of the earth's sphericity, it will not. Charles. Have not eclipses been esteemed as omens presaging some direful calamity? Tutor. Till the causes of these appearances were discovered, they were generally beheld with terror by the inhabitants of the world, which is beautifully alluded to by Milton in the first book of Paradise Lost, line 594: -As when the sun, new risen, Looks through the horizontal misty air CONVERSATION XXXVII. Of the Tides. Tutor. We will proceed to the consideration of the tides, or the flowing and ebbing of the ocean. James. Is this subject connected with astronomy? Tutor. It is, inasmuch as the tides are occasioned by the attraction of the sun and moon upon the waters, but more particularly by that of the latter. You will readily perceive that the tides are dependent upon some known and determinate laws, because, if you turn to the Ephemeris, or indeed to almost any almanac, you will see that the exact time of high water at Londonbridge for every day in the year is set down. Charles. I have frequently wondered how this could be known with such a degree of accuracy: indeed there is not a waterman that plies at the stairs, but can readily tell when it will be high water. Tutor. The generality of the watermen are probably as ignorant as yourself of the cause by which the waters flow and ebb, but by experience they know that the time of high water differs on each day about three quarters of an hour, or a little more or less, and therefore if it |