limited in its performance to short flights over prepared ground at a few metres height, ready to come to earth at a moment's notice; it will rather seek safety in altitude, probably flying in most part at a height of at least two or three thousand feet, where in the event of any minor mechanical failure or other hitch the time of gliding descent will be at the disposal of the aeronaut for adjustment, or, if it is found necessary to land, for the selection of a suitable site. Thus, if an aerodrome be flying one mile above the surface of the earth, there will be a circle of some ten or twelve miles diameter available within which to choose a place to alight; or, if we assume the velocity of flight to be forty miles per hour, the aeronaut will have at his disposal a period of some eight or ten minutes before he need finally come to earth.
The manner in which the industries connected with locomotion have proved mutually helpful is very striking; the dependence of the march of progress on such mutual assistance, nearly always traceable in the evolution of mechanism, is perhaps nowhere more apparent. Thus the modern automobile may almost be said to owe its existence to the pneumatic tyre, an invention whose utility was first established by the bicycle; and, in turn, the flying-machine has only become possible through the development of the internal combustion engine in the hands of the automobile engineer. It is evident that the market for the light weight petrol motor has grown up as a direct consequence of the demand for high speed, a demand that could not have arisen in the absence of the pneumatic tyre; hence even the old "hobby horse" may be regarded as one of the stepping stones to the conquest of the air, and we are led to regard the flying-machine as marking a great step in a vast evolutionary movement in locomotion, rather than as constituting in itself an independent invention.
The title of the present volume, "Aerodonetics," is one of two alternatives suggested in the preface to Vol. I., chosen as being from its derivation the more appropriate. The arrangement is as follows:—
