Cayley's work is difficult to assess. He had all the right ideas, though the means of putting them into practice did not lie ready to his hand. If he had been a poor man, he might have gone farther. He designed, so to say, both an airship and an aeroplane; there was no one to execute his designs, and the scheme fell through. He more than once anticipated later inventions, but he put nothing on the market. His mind was fertile in mechanical devices, so that if one proved troublesome, he could always turn his attention to another. He is content to enunciate a truth, and to call it probable. 'Probably', he says, in discussing engines of small weight and high power, 'a much cheaper engine of this sort might be produced by a gas-light apparatus and by firing the inflammable air generated with a due portion of common air under a piston.' This is an exact forecast of the engine used to-day in all flying machines. He has some good remarks on the shape that offers least resistance to the air in passing through it, that is, on the doctrine of the streamline. He knew that the shape of the hinder part of a solid body which travels through the air is of as much importance as the shape of the fore-part in diminishing resistance. He does not seem to have known that it is of more importance. He knew that the resistance of the air acting on concave wings, or planes, at a small angle of incidence was resolved chiefly into lift, and he suspected that the amount of the lift was greater than the mathematical theory of his day allowed. Above all, his treatise is stimulating, and suggests further inquiry and experiment along lines which have since proved to be the right lines.