72 THE SUN. 



of the sun, or what is the same, the mass of matter composing it, is three hun- 

 dred and fifty thousand times greater than the the mass of matter or weight 

 of the earth. 



To make a globe as heavy as the sun, it would then be necessary to agglom- 

 merate into one three hundred and fifty thousand globes like the earth 



DENSITY OF THE SUN. 



Having ascertained the weights and bulks of the bodies of the universe, we 

 are in a condition to determine their densities, and thus to obtain some clue to a 

 knowledge of their constituent materials. We have seen that while the bulk 

 of the sun is about one million and four hundred thousand times greater than 

 that of the earth, its weight is greater in the much less proportion of three hun- 

 dred and fifty thousand to one. Let us see to what inference this leads in re- 

 gard to the nature of the matter that composes the sun. If the materials of the 

 sun were similar to those of the earth, its weight would necessarily be greater 

 than that of the earth in the same proportion as its bulk, and in that case, of 

 course, the weight of the sun would be one million and four hundred thousand 

 times that of the earth. But it is not nearly so great as this ; on the contrary, 

 it is much less. Consequently, it follows that the constituent materials of 

 the sun are lighter than those of the earth in the proportion of about four to 

 one. The density of the sun is, therefore, very nearly equal to that of water, 

 and, consequently, the weight of the solar orb is equal to the weight of a globe 

 of the same magnitude composed altogether of water. 



FORM AND ROTATION OF THE SUN. 



Although to minds unaccustomed to the rigor of scientific research, it 

 might appear sufficiently evident, without further demonstration, that the sun 

 is globular in its form, yet the more exact methods pursued in the investiga- 

 tion of physics demand that we should find more conclusive proof of the sphe- 

 ricity of the solar orb than the mere fact that the disk of the sun is always cir- 

 cular. It is barely possible, however improbable, that a flat circular disk of 

 matter, the face of which should always be presented to ills earth, might be 

 the form of the sun ; and indeed there are a great variety of other forms which, 

 by a particular arrangement of their motions, might present to the eye a circu- 

 lar appearance as well as a globe or sphere. To prove, then, that a body is 

 globular, something more is necessary than the mere fact that it always appears 

 circular. 



When a telescope is directed to the sun, wo discover upon it certain marks 

 or spots, of which we shall speak more fully presently. We observe that 

 these marks, while they preserve the same relative position with respect to 

 each other, move regularly from one side of the sun to the other. They disap- 

 pear, and continue to be invisible for a certain time, come into view again on the 

 other side, and so once more pass over the sun's disk. This is an effect which 

 would evidently be produced by marks on the surface of a globe, the globe 

 itself revolving on an axis, and carrying these marks upon it. That this is, in 

 fact, the case, is abundantly proved by the fact that the periods of rotation for 

 all these marks are found to be exactly the same, viz., about twenty-five and a 

 half days. Such is, then, the time of rotation of the sun upon its axis, and that 

 it is a globe remains no longer doubtful, since the globe is the only body which, 

 while it revolves with a motion of rotation, could always present the circular 

 appearance to the eye. The axis on which the sun revolves is very nearly 

 perpendicular to the plane of the earth's orbit, and the mot' .n of rotation of the 



