I propose to deal with the subject of this paper under three headings : First, a brief summary of Cayley's life and character; second, an account of his work on navigable balloons; and third, a like account of his work on mechanical flight. The latter sections will be prefaced with a short survey of the state of those two branches of aeronautical science at the period of Cayley's youth. It should, perhaps, be added by way of introduction, that Cayley's place as a pioneer in .aeronautics has not yet been fully assessed. His name is barely mentioned in that greatly over-rated, ill-arranged and cumbrous compilation, “ Astra Castra,” published by Hatton Turnor in 1865, and it is entirely omitted from the “ Dictionary of National Biography.”