The presence of high buildings or other violent inequalities will also affect the density of the air and in turn the pressure exerted on the wings of an aeroplane. Among aviators it is generally believed that a great city is one of the most dangerous possible objects to fly above. In the case of New York with its many sky-scrapers, for instance, the danger is vastly increased. Even in the open country the presence of a deep valley or other depression will so affect the density of the atmosphere that an aeroplane is likely to be drawn down from its course. These areas are known among aviators as "pockets," and are often large enough to swallow up a large man-carrying craft, at times with disastrous effects. The chance of your model aeroplane running into such a pocket is of course considerable. A striking illustration of the effects of these chance currents was afforded during a recent model aero tournament in New York. A model aeroplane which had flown with remarkable steadiness for more than 150 feet chanced to pass over the head of a boy who was walking slowly across the course. This moving object served to set up a small whirlpool of air. The model on striking it was instantly checked, when it turned, skirted the column of air and passed on. In an indoor flying, an open window, merely by changing the temperature slightly in its vicinity, will often cause a model to be seriously deflected, perhaps to be thrown completely off its course. It will often be noticed in outdoor flying that a model, in passing over a stream or body of water or a mass of dense foliage, will encounter a change of temperature which will appreciably affect its course. In flying model aeroplanes the performance of a machine will often vary unaccountably from day to day. With the same motor and winding, the model will fly much higher and more freely at different seasons of the year. It is well to bear in mind that in the summer months the heat causes a low density. The pressure exerted by the atmosphere is therefore correspondingly small; the model, or for that matter a large machine, travels much faster. A dry day also tends to cause low density. This will account for the excellent flights on warm, dry days and the crankiness of the machine when the weather is damp. Cold weather on the other hand tends to increase the density of the air. The speed of an aeroplane is materially reduced but on the other hand the air will be found more buoyant. Such weather is better for heavy models, although they fly much more slowly. It will be seen that at high altitudes, where the density is least, models should fly both faster and higher than they will nearer the sea level.