31$ PRACTICAL LESSONS IN SCIENCE. 



The Planetoids or asteroids are a group of smaller bodies re- 

 volving around the sun between the orbits of Mars and Jupiter. 



The Moons or satellites are bodies which revolve around some 

 of the planets as these revolve around the sun. The Earth has 

 one moon, Mars two, Jupiter four, Saturn has eight moons and 

 in addition two rings, Uranus has four or more satellites, and 

 Neptune at least one. We can know but little about the moons 

 but they all seem to be spherical bodies and in general to revolve 

 in the same direction as their primaries. The satellite of the 

 Earth, one of the largest of the moons, is nearly a perfect sphere 

 having a diameter of 2,160 miles. It rotates on its axis and re- 

 volves around the earth in about twenty-seven and one-third 

 days, so that the same side is always toward the earth. Seen 

 through a telescope the surface of the moon appears to be very 

 rugged, not from mountain ranges but from great circular de- 

 pressions in which there are sometimes conical elevations. The 

 moon seems to be a great cinder-like body, devoid of air, water 

 and every form of life. It is supposed that the rocky mass may 

 have absorbed the air and water which were formerly abundant 

 on its surface. 



The Comets are interesting, but little known, members of the 

 solar system. They are usually rather small bodies with an ex- 

 tensive atmosphere, revolving in much elongated orbits, moving 

 with great velocity when near the sun. 



Meteors are bodies, which, falling toward the earth, are heated 

 to incandescence by friction in the air, and usually vaporized be- 

 fore reaching the earth. Sometimes they reach the earth as me- 

 teoric stones or meteorites. In general the meteorite is composed 

 mainly of iron, sometimes of a stony substance. 



The Stars are supposed to be bodies like our Sun, many of 

 them, however, much larger than it. The spectroscope shows 

 that many of them contain substances found to exist in the sun 

 and well known on the earth. The stars vary in size, in distance 

 from the earth, in temperature, in color, and apparently in com- 

 position as well. Some appear to be double stars, some revolve 

 about others ; others are variable in appearance, often changing 



