The Lesser Members of our System--Bode's Law--The Vacant Region in the Planetary System--The Research--The Discovery of Piazzi--Was the small Body a Planet?--The Planet becomes Invisible--Gauss undertakes the Search by Mathematics--The Planet Recovered--Further Discoveries--Number of Minor Planets now known--The Region to be Searched--The Construction of the Chart for the Search for Small Planets--How a Minor Planet is Discovered--Physical Nature of the Minor Planets--Small Gravitation on the Minor Planets--The Berlin Computations--How the Minor Planets tell us the Distance of the Sun--Accuracy of the Observations--How they may be Multiplied--Victoria and Sappho--The most Perfect Method. In our chapters on the Sun and Moon, on the Earth and Venus, and on Mercury and Mars, we have been discussing the features and the movements of globes of vast dimensions. The least of all these bodies is the moon, but even that globe is 2,000 miles from one side to the other. In approaching the subject of the minor planets we must be prepared to find objects of dimensions quite inconsiderable in comparison with the great spheres 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 almost all invisible. In the diagram (p. 234) of the orbits of the various planets, it is shown that a wide space exists between the orbit of Mars and that of Jupiter. It was often surmised that this ample 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. Take the series of numbers, 0, 3, 6, 12, 24, 48, 96, whereof each number (except the second) is double of the number which precedes it. If we now add four to each, we have the series 4, 7, 10, 16, 28, 52, 100. With the exception of the fifth of these numbers (28), they are all sensibly proportional to the distances of the various planets from the sun. In fact, the distances are as follows:--Mercury, 3.9; Venus, 7.2; Earth, 10; Mars, 15.2; Jupiter, 52.9; Saturn, 95.4. Although we have no physical reason to offer why this law--generally known as Bode's--should be true, yet the fact that it is so nearly true in the case of all the known planets tempts us to ask whether there may not also be a planet revolving around the sun at the distance represented by 28.