The castle had always a well for water; stables for horses, and room for their food; it had places for soldiers; dungeons for prisoners; and apartments for many hundreds of people. In time of war, all the people round about would flock to the castle, and there they would live, sometimes for months. Castles were usually built upon lofty pieces of ground, the access to which was very difficult. These were generally surrounded by deep ditches, filled with water, across which an enemy could not easily pass. Over this was a bridge for the use of the people of the castle, which was taken away whenever any fear of an enemy was entertained. Castles had towers, generally at the four corners, and sometimes each side of the gate. These were of a circular form, and had loopholes, like narrow windows, out of which the soldiers within, shot their arrows. Sometimes a castle would be besieged for months, by armies encompassing it. The army without would try every means to get into the castle, or kill the people. They would shoot their arrows with cross-bows, and endeavor to knock down the walls with battering-rams. In travelling over Europe, the ruins of many castles are still to be seen. Few are met with which are in good repair, as over all Europe the people are now governed by laws, which afford protection, without resorting to castles. These are, therefore, but little used at the present day; and even in cases where they are still occupied, it is only as dwellings, and not as strong-holds for safety against enemies. [Illustration: _Manner of attacking a castle in the middle ages._] Æronautics, OR ART OF NAVIGATING THE AIR WITH BALLOONS. Although this subject has little _practical_ importance, yet, in view of its interest, especially to the young reader, we shall devote a few pages to a notice of some of the most splendid ærial voyages which we find on record. The fundamental principles of this art have long been known; but the application of them to practice is a modern discovery. About the year 1765, Mr. Henry Cavendish made his celebrated discovery of the existence of inflammable air, or, as it is called, hydrogen gas. Common air is eight hundred times lighter than water; hydrogen gas seven times lighter than common air. This is the material now used for the purpose of filling balloons. It is obtained in several ways; but the best methods are by applying acids to certain metals. Iron, zinc and sulphuric acid (oil of vitriol) are most commonly used. The acid must be diluted with five or six parts of water. Iron may be expected to yield in the common way about seventeen hundred times its own bulk of gas, or four and a half ounces of iron, the like weight of sulphuric acid, and twenty-two and a half ounces of water, will produce one cubic foot of hydrogen gas; six ounces of zinc, an equal weight of acid, and thirty ounces of water, are necessary for producing the same quantity.