The title of this paper does not immediately suggest aeronautics, but the connection is very intimate, as the laws of similitude constitute the theory of the use of models. Aeronautics, or rather, aerodynamics, is almost entirely an experimental science at present, and a glance backwards thiough time will show how much of the fundamental data of the subject is derived from experiments on models. We have, in the first place, Langley's researches, which showed that sufficient lift could be obtained to make models fly with such light engines as he was able to construct. The development of the petrol motor removed the greater part of the engine difficulty, and with that out of the way, Langley's figures for model planes, modified by Lilienthal's experiments on cambered models, provided the initial information from which the earliest flying machines were constructed.