§ 18. While the Moon is moving round the Earth, it often occurs that she is placed in a direct line between ourselves and the Sun. In this case a greater or less part of the Sun is concealed from us, which causes a diminution of light or a partial darkness on our Earth. This we call an _Eclipse of the Sun_. (Such an Eclipse took place in 1831, and you will probably have an opportunity of seeing many more). If, on the contrary, the Earth is placed in a direct line between the Sun and the Moon, then the Moon will be obscured by our Earth. This is called an _Eclipse of the Moon_. The following two figures on Plate No. VIII will serve for an illustration. You will easily perceive from them that if the Moon (as represented in Figure I) is placed in a direct line between the Sun and ourselves, it must necessarily conceal from us part of that luminary; and in this state cast a _shade_ upon our Earth. But if the _Earth_ is placed in a direct line between the Sun and the Moon, (as represented in Figure II), then the Moon will be much more obscured, because the Earth is much larger than the Moon, and will therefore cast a much greater shade upon her. § 19. It remains for us to speak of that class of bodies known by the name of _Comets_, (see Lesson I, § 6). Of these an unknown number belongs to our Solar System.—(Some philosophers have estimated their number to be about Twentyone; others think it must amount to several hundred). They move round the Sun in exceedingly long ovals, having their transparent tails always turned away from that luminary. What is most remarkable about them is the astonishing degree of heat to which they are exposed on account of passing so near the Sun, and the astonishing velocity with which they travel. The Comet which appeared in the year 1680, is supposed to sustain a heat nearly Two Thousand times greater than that of red hot iron, and to move at the rate of several Hundred Thousand miles an hour!! RECAPITULATION OF LESSON III. QUESTIONS. [§ 14.] Do all Planets need the same time to complete a whole revolution round the Sun? Why not? If the pupils are old enough to understand the use of Dividers, it will perhaps be well for the teacher to let them draw the Solar System on a piece of paper.—If not, he ought to let them explain Plate IV, or an orrery, if one be at hand. [§ 15.] What is the time called, which our Earth needs for a complete revolution round the Sun? How many days are there in a year? How many hours are there in a day? What is the revolution of the Earth round the Sun, the means of? [§ 16.] What time does Mercury require for a complete revolution round the Sun? What time does Venus require for the same purpose? What time does Ceres, Vesta, Juno and Pallas need? What, Jupiter, Saturn, and Herschel?