In his _Recherche sur le Feu Grégeois_, p. 23, M. Chrétien-Lalanne maintains that the incendiary was expelled from the siphon by means of a spring. This theory is inadmissible, for helical springs are not heard of until long after the time in question. Further, the ancients possessed no means of condensing air to the degree necessary for the projection of a heavy body over even the short ranges of the Dark Ages, and steam power had hardly been recognised at all.[93] Therefore, it has been urged, the incendiary must have been expelled from the siphon by means of an explosive saltpetre mixture, this being the only way of effecting the object that remained at the disposal of the ancients. As will be seen presently, there remained a simple and efficacious method, involving very little expense and no danger whatever; a fact which in itself would be sufficient to meet the above argument in favour of saltpetre, even were it unsupported by the evidence already brought forward in Chap. ii. to show that saltpetre had not been yet discovered at the time in question, and the evidence adduced in the present chapter to prove that in fact it was not used. Further, the supposition that an explosive was employed is excluded by the construction of the siphon, which was made of wood. Such is the only reasonable explanation that can be given of the Emperor Leo’s order that the siphons should be “cased with bronze.”[94] Had they been of metal, a casing of bronze would have been a useless complication; but, being of wood, an internal casing of metal was absolutely necessary to protect them from the flame of the burning composition. Only one round probably could have been fired from a wooden tube by means of an explosive, and that round in most cases would have been more fatal to the siphon detachment than to the enemy. Again, as the projectile was simply a lump of oleaginous matter, it would have been blown by an explosive cartridge into thousands of fragments, each of them so small as to be worthless for incendiary purposes; for the efficacy of an incendiary depends to a great extent on its containing a large quantity of matter. Since the use of springs, compressed air, and steam were impossible, and the use of an explosive extremely improbable, it only remains to examine the arguments for and against water as the motive power.