As the roc has proved a myth, one questions whether a saddle bird may not be evolved by judicious breeding. But opposed to this is the square-cube law of the Greek geometer, by which a learned geologist demonstrated that nature has reached the limit of her resources in the production of large flyers, the ostrich, for example, being too bulky to navigate at all. As a last resource, then, the human dwarf may breed his weight downward to accommodate the bird. Assuredly, the most powerful flyer can carry the lightest human dwarf without difficulty. Such aërial cavalry has been projected occasionally, and if fairly developed might have interesting employment. Its military value, to say nothing of its civil uses, would be considerable. An aërial scout that could hide in a tree top, or small cloud, then flit home with full intelligence of the enemy, would be effective and unique. In aggressive warfare it would serve the plan of that ingenious Englishman who proposes to repel a German invasion by dispatching birds to peck holes in the enemy’s war balloons. But here the dwarf might be omitted, if the birds were taught to have a definite interest in attacking aërial cruisers with their beaks, or with steel-armed spurs like those of the Spanish fighting cock, or with talons treated chemically to strike fire. Sparrows with sulphur-pointed toes could easily annihilate an aërial squadron at all combustible. Recurring to the geologist, it may be added that, having discovered the major limit of feathered navigators, he concluded, as a corollary, that human flight is forever impossible. That was in the latter eighties. In 1901 a versatile astronomer adduced the same law to prove that an aëroplane could not be made to carry a man. Presently, learning that this had been achieved, he proved, in a second mellifluous paper, that an aëroplane could not carry, several men.[4] Having erred twice, he wrote a final article announcing that a flyer is fatuous, anyhow, because she cannot repair her engines in the sky! [Illustration: FIG. 2.—A POSSIBLE AIR-SCOUT.]