It is not generally realized by the laymen how rapid has been the development of "aerology," nor how practical are the results obtained. There have already been established in Germany three scientifically equipped stations for observing air conditions for the benefit of aeronauts, just as the weather bureau observes weather conditions and informs ships at sea of approaching storms. One of these, established by Dr. Polis at Aachen, is now in operation. In addition to this the Public Weather Service Stations of Germany have been equipped recently with the necessary apparatus for making daily observations of the upper air for the special benefit for aerial navigators. The resistance offered by the air to the passage of an air ship of any type depends upon its density. The air is obviously an exceedingly variable medium, as capricious as quicksilver, as both the sky pilot and the flyer of model aeroplanes have learned. The density and therefore the resistance depends upon the temperature, the pressure and the state of equilibrium. We are in the habit of thinking of density and pressure as affecting enormous volumes or areas miles in extent, such as are reported in the weather forecasts from day to day. To an extent these same conditions are found changing within a few feet. It is this tendency to rapid change of conditions which makes the problem of stability in aeronautics so baffling. The pressure upon an aeroplane, whether a man-carrying machine or a model, varies considerably between the level of the seashore and the top of a mountain. An aeroplane in rising from the level of the sea for several thousand feet therefore meets new and unexpected conditions. The density is reduced fully one-half at an altitude of 18,000 feet, and since aeroplanes have risen more than 10,000 feet this must be taken into consideration. Even at a height of 300 feet, which is often reached by aeroplanes in flight, the difference in the pressure calls for skilful manipulation on the part of the sky pilot. The density varies again according to the temperature. Let an aviator suddenly run into a hot or a cold strait of air and the pressure upon his planes will instantly change. The effect of temperature must be taken into consideration by any one flying model aeroplanes. It may happen that a draft of cool or of hot air, by changing the pressure on the planes, will throw a model aeroplane out of balance and mar an otherwise promising flight. A difference of a few degrees of temperature will often affect a very sensitive model. The only way to combat this is of course to build your model with the greatest possible stability.