Other volcanoes behave quite differently from these violent volcanoes, and do hardly any noteworthy damage. Among these is the crater-island of Stromboli, situated between Sicily and Calabria. This volcano has been in continuous activity for thousands of years. Its eruptions succeed one another at intervals ranging from one minute to twenty minutes, and its fire serves the sailors as a natural light-house. The force of this volcano is, of course, unequal at different periods. In the summer of 1906 it is said to have been in unusually violent activity. Very quiet, as a rule, are the eruptions of the great volcanoes on Hawaii. Foremost among the substances which are ejected from volcanoes is water vapor. The cloud floating above the crater is, for this reason, the surest criterion of the activity of the volcano. Violent eruptions drive the masses of steam up into the air to heights of 8 km. (5 miles), as the illustrations (Figs. 1 to 4) will show. The height of the cloud may be judged from the height of Vesuvius, 1300 metres (nearly 4300 ft.) above sea-level. The illustration on page 4 (Fig. 2) is a reproduction of a drawing by Poulett Scrope, representing the Vesuvius eruption of the year 1822. There seems to have been no wind on this day; the masses of steam formed a cloud of a regular shape which reminds us of a pine-tree. According to the description of Plinius, the cloud noticed at the eruption of Vesuvius in the year 79 must have been of the same kind. When the air is not so calm the cloud assumes a more irregular shape (Fig. 3). Clouds which rise to such elevations as we have spoken of are distinguished by strong electric charges. The vivid flashes of lightning which shoot out of the black clouds add to the terror of the awful spectacle. [Illustration: Fig. 2.—Eruption of Vesuvius in 1882. (After a contemporaneous drawing by Poulett Scrope)]