heavens, which is due to the attraction of the different planets nearly all the variations which appear in his dimensions arise from the motion of the planet on which the spectator is situated, and not from the motion of the Sun. But, although the motion of the Sun in an orbit is very small, never extending beyond the length of the solar diameter, yet he has another species of motion quite independent of any change of place; we mean a motion round his axis, which is completed in twenty-five days. The means by which this motion is made manifest, are the disappearance and re-appearance of certain dark or shaded spots, on his brilliant surface. These spots we shall have to allude to further on; but we may now state, that certain dark spots appear to travel, as it were, across the sun,-disappear at the western edge, become lost for a time to the view,-re-appear at the opposite or eastern edge, and then attain their former position. This can only occur on the supposition, that the Sun revolves on an axis; and the period of such revolution, as determined by these spots, after making allowance for the motion of the Earth, is, as we have stated, about twenty-five days. It is easy to produce familiar illustration of such a motion. Suppose that we have an orange, with a dark spot on one part of its surface: if we turn the orange round on its axis, that is, in the way that a top spins, we shall find that the dark spot will gradually reach one edge of the visible part of the round on the remote side,-and re-appear orange,-pass on the opposite edge from that at which it previously disappeared.

The apparent size of a body to a spectator depends on two circumstances:-First, the real size of the object;

and, secondly, the distance at which it is placed from the observer. Now, as we may reasonably suppose that the Sun's dimensions are constant, any variation in his apparent size must be due to distance. It may now be asked by one who has not previously thought of these subjects, how we can describe in words, the size of which the Sun appears to us? If we say that he looks as large as an orange, no definite information is conveyed to the mind by such a statement; because, if an orange be held at a few inches distance from the eye, it will appear much larger than the Sun; whereas, if it be held at some yards distance, it will appear smaller than the Sun. It is obvious, therefore, that we must devise some other mode than comparisons of this sort, if we wish to convey to others an idea of the size of an object.

The mode which is adopted for this purpose, is to express the angle, which the diameter of a body subtends, or is opposite to. As we shall frequently have to speak of the word angle, it may be desirable to explain in this place what is meant by that term, and how angles are measured. Suppose that we have a stick, or any other object, which we wish to place upright, on one of its ends, on a table. We know that, unless it

and by the word upright,

be quite upright, it will fall; we mean that it inclines neither one way nor the other, Now it has been agreed that we shall call the position which the stick thus occupies with respect to the table, by the name of a right angle. Thus, (fig. 9,) if a c be the stick standing upright on the table C B, the opening or space included between the stick A c, and the table B C, is called a right angle. Suppose now that we lean the stick into the direction denoted by the dotted line

DC: then the opening or space included between the stick and the table will be smaller than before. When we incline the covers of a book towards each other, in the act of closing the book, we produce a similar effect to that which we have just noticed;—that is, we make two lines, or two surfaces (as the case may be,) close towards each other, as if one were about to lie on the other. This effect, then, is expressed, by saying that the angle D C B is smaller than the angle A C B. (We always place the letter attached to the point of junction of the two lines, between the two letters indicating the other ends of those lines.) If the stick were still more

Fig. 9.

B

inclined, so as to occupy the position E C, then it would appear to approach still nearer to the table; the space included between them would be smaller; and we should say that the angle E C B is smaller than either of the former angles. It is observable that, in this way