Down to the time of Robins, gunnery was still held to be an art and a mystery. The gunner, that honest and godly man,[80] learned in arithmetic and astronomy, was master of a terrible craft;--his saltpetre gathered, it was said, from within vaults, tombs, and other desolate places;--his touchwood made from old toadstools dried over a smoky fire;--himself working unscathed only by grace of St. Barbara, the protectress of all artillerymen. The efficiency of his practice depended overwhelmingly on his own knowledge and on the skill with which he mixed and adjusted his materials. No item in his system was of sealed pattern; every element varied between the widest limits. There were no range-tables. His shots varied in size according to the time they happened to have been in service, to the degree of rusting and flaking which they had suffered, and to their initial variations in manufacture. His piece might be bored taper; if so, and if smaller at the breech end than at the muzzle, there was a good chance of some shot being rammed short of the powder, leaving an air space, so that the gun might burst on discharge; if smaller at the muzzle end the initial windage would be too great, perhaps, to allow of efficient discharge of any shot which could be entered. There was always danger to be apprehended from cracks and flaws. But the greatest of mysteries was that in which the flight of projectiles was shrouded. At this point gunnery touched one of the oldest and one of the main aspects of natural philosophy.