In Dr. Assmann's plan, a number of the Public Weather Service stations are to be furnished by the Lindenberg Observatory with a theodolite, an inflating-balance for determining the ascensional force of the balloons, a sufficient number of balloons, and the necessary graphic tables for rapidly working up the observations.

At 8 a. m. every day, assuming the weather is favourable, the stations will be expected to send up a pilot balloon and to trace its course with the theodolite as long as possible. The observation will then be worked up — a matter of barely a quarter of an hour for a practised observer — and telegraphed to Lindenberg.

Fig. 48. Henry Farman seated in his biplane with three passengers.

Photograph by Edwin Levick.

Here the observations received from all other stations will be assembled and re-distributed in a single telegram sent to each of the cooperating stations. If they arrive in time, the telegrams can be utilised in connection with the ordinary daily weather forecast, as well as for the preparation of special forecasts and warnings for airmen. At Lindenberg the regular observation with a kite or capture balloon is made daily at 8 a. m., and in summer an observation is also made about 5 or 6 a. m. Assmann also proposes to conduct daily observations at Lindenberg with a pilot balloon at 11 a. m., and, whenever necessary, another about 2 p. m., so that soundings of the air to an altitude of several miles will be made three or four times a day within a period of six to nine hours. Thus valuable information will be gathered which ought to enable the weather forecaster to warn airmen of impending changes in the lower atmosphere, on the basis of actually occurring rapid changes in the upper atmosphere.

That the German Public Weather Service stations will ultimately be supplemented with stations especially erected for the purpose at the larger aviation fields and the like, would seem to follow from the work now done at the experimental observatory at Bitterfeld, from the erection of the aeronautical observatory on the Inselberg, near Gotha, from the probability of the erection of the long-promised aerological station at Taunus, and lastly, from the contemplated installation of aerological stations at nautical schools on the coast.

The difficulty of following pilot balloons in hazy weather and at dusk leads Assmann to propose the utilisation of balloons of two sizes, the smaller and cheaper to be used when it is evident that the state of the sky will not permit the balloon to be followed with the theodolite to a great distance. Observations at night could be made by illuminated balloons, but at considerable expense.

Undoubtedly there will be many days on which few, if any, observations can be secured with pilot balloons, so that only the observations made at stations equipped with captive balloons and kites will be available. In order to meet this serious difficulty, Assmann is considering the plan of supplying a few selected stations with a central and easily manageable kite outfit.

Thus far the plan outlined by Dr. Assmann has been approved only for a limited part of the Empire. Political heterogeneity still hampers imperial undertakings in Germany. Ultimately, however, the field of observations will be extended to include the south German states, where some very important stations are located, chief among which is the admirably equipped station at Friedrichshafen on Lake Constance.

Assmann himself realises that his plan cannot hope to provide detailed information and forecasts of local conditions except in so far as may be inferred from the general outlook. Some experiments which were recently made in Germany, after the appearance of Dr. Assmann's article, show that it is feasible to secure a corps of special thunderstorm observers who can report by telegraph and telephone, and who are numerous enough to enable the weather forecaster to follow the progress of sudden atmospheric disturbances across the country, and to give timely warning to the aeronaut to avoid them.

Apart from enlightening the aeronaut on the condition of the atmosphere, it will be obviously necessary to provide the equivalent of automobile road maps, — something that will tell the man of the air where he is. It is very difficult to recognise even familiar country from above. During his flight down the Hudson River, Cur-tiss decided to alight on what looked to him like a fine green field. Swooping down, he found that his green field was a terrace, an unavoidable error in judgment which might have cut short his triumphal flight. With a map on a scale of half an inch to the mile, showing the lines of the roads and the shapes of the villages, it would seem easy enough to ascertain one's whereabouts; but the aviator travels quickly and a full equipment of half-inch maps would be a serious item in the weight of his load. The man in a balloon is often above clouds, and when he views the earth again it is very difficult and frequently impossible to pick up the route again. The aviator in a flying-machine is more favourably placed. He knows his direction approximately, although he is often unable to make proper allowance for the drifting effect of the wind. If caught by varying currents or by storms above the clouds, he easily loses track of the course. There will be need of large distinctive ground marks for day and lights for night at distances of ten miles apart, marks which will correspond with those on an air-chart. Zeppelin proposes maps showing heights by colours, and marks indicating the influence of streams, marshes, and woods on the static equilibrium of the airship. The scale he suggests is three miles to the inch. Colour is the main consideration. In the opinion of Mr. Charles Cyril Turner, an English aeronaut, the colours should approximate to the colours of the landscape as seen from above. The roads should be white, the water blue, the fields light green, woods a darker green, habitations grey, and railways black.