THE TEACHER'S OFFERING.

The Sun.

THE Sun has attracted far more attention during the last few years than any other heavenly body, and in consequence several remarkable discoveries have been made on it well worthy of notice, some of which we will endeavour to describe.

The Sun is the central body round which the Earth and eighty-seven other planets, with hundreds of Comets, revolve. It is so large a body that if all the planets were brought together into one mass, they would not amount to the sixhundredth part of the size of the Sun. It would take one million, four hundred thousand globes the size of the Earth, to make one as large as the Sun. The real diameter, or thickness of this immense body, is eight hundred and eighty-seven thousand miles; so that a railway-train travelling thirty miles per hour night and day, would take one year and seven months to go once round it.

To the naked eye the Sun appears so intensely bright that

no markings can be distinguished on its surface, but if we employ a powerful telescope and look through a piece of very dark glass, or a small tube of ink and water, we shall see that it is mottled all over with many black spots and a large number of bright streaks crossing each other in various directions.

The spots vary very much both as to their number and the size of those that are visible. They generally last only a few weeks, but are soon succeeded by others, so that the sun's surface, or disc as it is called, is rarely free from them, and sometimes thirty or forty may be seen at once. Some spots are so large that they may be seen without a telescope by looking through a piece of smoked glass; when this is the case they must be at least two or three times the size of the Earth.

If we examine one of these spots carefully with the telescope, we shall find that it consists of two principal parts, a dark centre of a very irregular shape called the nucleus, surrounded by a border rather brighter and frequently streaked with bright lines, which is called the penumbra. By examining the nucleus still more closely we shall generally see a still darker point in the centre of it, so that on the larger spots there are three different shades or degrees of brightness, the darkest of which is in the centre.

Astronomers differ slightly in their opinions of the nature of these spots; the explanation most generally accepted is this: the Sun itself is supposed to be a dark

body similar to the Earth, but surrounded with several atmospheres or shells of vapour somewhat like the structure of Comets, which we noticed in the January number of the TEACHER'S OFFERING. The outer atmosphere is almost transparent, and has only recently been discovered. We shall have something to tell you about this another time.

The next atmosphere is the luminous one which produces the light and heat of the Sun, and within this there are probably two more,-one heavily loaded with clouds appearing as the penumbra of the spots, and the inner one as the dark shade round their centres. According to this theory, the spots are produced by whirlwinds on the surface of the Sun, which blow the luminous atmosphere on one side, and show us the dark body of the Sun through the opening thus made.

The cooling effects of these strata of clouds and air would be so great, that as a celebrated author says, “the Sun itself may now be viewed as a cool summer residence," although the heat of the luminous stratum is so great, that every substance with which we are acquainted would not only be melted, but actually converted into vapour.

What an insight this gives us into the boundless power of God, that a body so hot as to give light and heat to bodies hundreds of millions of miles distant, should yet be habitable to beings like ourselves.

The bright streaks or "facula" of the Sun have been brought into much notice lately through some observations

of Mr. Nasmyth, who has discovered that they cover the whole surface of the Sun excepting the dark spots. They present the appearance of a large number of streaks of light crossing each other in every possible direction. Mr. Nasmyth compares them to willow leaves, Mr. Dawes to pieces of straw or thatching, but with less powerful telescopes they appear only as irregular lines of light. They vary greatly in size, but probably average about one thousand miles in length, and one hundred in breadth.

The cause of these markings is very uncertain; they are evidently owing to a greater heat at these points than at other parts of the Sun's surface, but the way in which the heat is so increased is not clear.

An opinion has gained much favour lately, that they are formed by the combustion or burning of very small planets, which have been revolving around the Sun and gradually fall into it. Some such addition of fuel to the Sun must be necessary, or its heat and light would rapidly decrease, and the plants and animals on the planets which revolve around it would all perish. This subject, however, is still involved in much mystery.

There is just one subject more in connection with the Sun which we must mention, it is called "Spectrum Analysis;" a very hard name certainly, and it would take a long time to explain it fully, but we can tell you what it enables us to do. If we receive the light of the Sun into a telescope, and treat it in a peculiar way by means of prisms or three