We must now dismiss our tutors and try to walk alone as we pass on 1 from what is admitted to what is debatable in this subject. What is to be classed as admitted? It is admitted that the thunderbolt is fire of some kind; similarly with the lightning flash, which is simply flame ready to become a bolt if it had more strength. The difference between the two is not in character but in force. The fiery nature of the bolt is proved by its heat. Apart from that, its effects prove it, for it has often been the cause of great conflagrations. Forests and 2 portions of cities have been burnt to ashes by it. Even objects that are not struck are yet seen to be scorched, some are discoloured as if by smoky grime. Then, again, everything that lightning strikes has the smell of sulphur. And so it is beyond dispute that both phenomena are a form of fire, and that they differ merely in their method of movement. A flash is a bolt that has not strength to carry it down to the 3 earth. And conversely you may say that the bolt is a flash that has been conveyed right down to the ground. It is not for the purpose of refinement of terms that I deal at some length with them, but in order to prove the phenomena related and of the same category and character. A bolt is something more than a flash. Inverting the statement, a flash is all but a bolt. XXII Now that it is agreed that the two things are both fire, let us see 1 how fire arises on earth, for no doubt the same method prevails aloft. There are two common methods of producing fire--one by striking it out, as, for example, from a stone; the other by the more tedious method of friction, as when two pieces of wood are rubbed together for some time. It is, of course, not every kind of substance that gives the desired result; you must choose one suitable for giving out fire, for example, laurel, ivy, and other trees familiar to shepherds for this purpose. Probably, therefore, clouds may in the same way emit fire either from a blow or from friction. Consider for a moment the force with which squalls rush forth, the impetuous eddying revolution of 2 the whirlwind. Anything that encounters a missile from an engine of war is scattered and removed and driven far from its position. What wonder, then, that such violence in the wind extracts fire either from some external object or merely from itself? You can readily see what a glow all neighbouring bodies grazed by its passage must receive. But the force of storms cannot for a moment be compared with the energy of the heavenly bodies, whose immense power is beyond question. XXIII