In our chapters on the Sun and Moon, on the Earth and Venus, and on Mercury and Mars, we have been usually discussing the features and the movements of globes of vast dimensions. The least of all these bodies is the moon, but even our moon is a ball 2,000 miles from one side to the other. In approaching the subject of the minor planets∧ we must be prepared to find planets of dimension? quite inconsiderable in comparison with the great globes of our system. No doubt these minor planets are all of them some few miles, and some of them a great many miles, in diameter. Were they close to the earth they would be conspicuous, and even splendid objects; but as they are so distant they do not, even in our greatest telescopes, become very remarkable, while to the unaided eye they are totally invisible.

In a diagram of the orbits of the various planets, it is shown that a wide space exists between the orbit of Mars and the orbit of Jupiter. It was often surmised that this wide region must be tenanted by some other planet. The presumption became much stronger when a remarkable law was discovered which exhibited, with considerable accuracy, the relative distances of the great planets of our system.