But in both these cases the inquirers paid the penalty of having adopted a wrong method. If we wish to examine the mode of working of a complicated steam-engine, it will be of little avail for us to watch the machinery when the full blast of steam is turned on, and the rapid movements of levers, pinions, and slides baffle all attempts to follow them, and render hopeless every effort to trace their connection with one another. But if some friendly hand turn off the greater part of the steam-supply, then, as the rods move slowly backwards and forwards, as the wheels make their measured revolutions, and the valves axe seen gradually opening and shutting, we may have an opportunity of determining the relations of the several parts of the machine to one another, and of arriving at just conclusions concerning the plan on which it is constructed. Nor can we doubt that the parts of the machine bear the same relation to one another, and that their movements take place in precisely the same order, when the supply of steam is large as when it is small. Now, as we shall show in the sequel, a volcano is a kind of great natural steam-engine, and our best method of investigating its action is to watch it when a part of the steam-supply is cut off. It is true that we cannot at will control the source of supply of steam to a volcano, as we can in a steam-engine, but as some volcanoes have usually only a small steam-supply, and nearly all volcanoes vary greatly in the intensity of their action at different periods, we can, by a careful selection of the object or the time of our study, gain all those advantages which would be obtained by regulating its action for ourselves. Spallanzani appears to have been the first to perceive the important fact, that the nature of volcanic action remains the same, however its intensity may vary. Taking advantage of the circumstance that there exists in the Mediterranean Sea a volcano--Stromboli--which for at least 2,000 years has been in a constant and regular, but not in a violent or dangerous, state of activity, he visited the spot, and made the series of careful observations which laid the foundation of our knowledge of the 'physiology of volcanoes.' Since the time of Spallanzani, many other investigators have visited the crater of Stromboli, and they have been able to confirm and extend the observations of the great Italian naturalist, as to the character of the action which is constantly taking place within it. We cannot better illustrate the nature of volcanic action than by describing what has been witnessed by numerous observers within the crater of Stromboli, where it is possible to watch the series of operations going on by the hour together, and to do so without having our judgment warped either by an excited imagination or the sense of danger. [Sidenote: APPEARANCE OF STROMBOLI FROM A DISTANCE.]