One of the curiosities of the air is the effect of eddies created by the passage of aeroplanes. Let an aeroplane move fast enough through the air and it will create high density above its course, and low density below its line of flight. This condition may also be found in the wake of model aeroplanes. It will often be noticed when two models are flying near together that the disturbance caused by one will seriously interfere with the second machine. The contrast between the air above and below the models in flight in especially noticeable when the planes are flexed. [Illustration: Two of the earlier Peoli models] A great deal has been learned of the action of the air upon aeroplanes by photographing the air currents. The smallest eddies of the air have been made visible by taking instantaneous photographs of thin smoke as it passes obstacles of various size and form. It has been found that a square object causes a great deal of disturbance when in an air current. The air is compressed in front of it and eddies for some distance in its wake before finally coming to rest. An elliptical object causes less disturbance, but the air continues to splash in its wake for some distance. Even a perfectly spherical object offers a surprising amount of resistance to the air. In the case of a long narrow ellipse the disturbance is considerably reduced. A curved surface however, such as is used for the planes of an aeroplane, cuts the air with practically no resistance, and the air flows smoothly about and joins behind it with very little wake or splashing. A form which suits the air in this way or a "stream line" body as it is called is obviously just the right design for the wings of an aeroplane. Even the experts in aeronautics differ so widely, however, that it is impossible to lay down any definite rules. One of the greatest authorities on the science of aviation, Mr. Horatio Philips, of England, believes that aeroplanes gain more support from the entering edges of their planes than from the rest of their surface. He argues that it is this edge, meeting the air currents, which serves to hold the aeroplane suspended. This theory is founded on the experiments made with a machine of original design flown by Mr. Philips as far back as 1890. It consisted of a series of planes mounted one above the other at regular intervals, much the same as the strips of a Venetian blind. The lifting power of this model in proportion to its surface and the power exerted was enormous. This theory is borne out in part by the success of the model aeroplanes with very narrow wings which have been flown with great success during the year. The beautiful model built by Stewart Easter, for instance, which is illustrated on another page, depends for its support on planes which are no wider than ordinary window blinds.