A less perilous voyage northward across the Alps was attempted in 1898 by Professor Heim, the Swiss geologist, and two associates, conducted by the Italian aeronaut, Spelterini. With an automatic photographic camera, similar to one described in the next chapter, it was hoped to get views of the high Alps from above, which would be alike valuable for geologic and topographic study. Extensive meteorological observations were made in connection with the sixth international balloon ascent, but only the Jura was crossed, at an altitude of 13,000 feet, because the balloon travelled in a north-westerly direction, instead of north-east as was expected. Many years ago Wise and Donaldson, the American aeronauts, proposed to cross the Atlantic Ocean in a balloon. The difficulties which present themselves in such an undertaking are purely technical, and given a balloon which loses its gas so slowly that its buoyancy can be maintained for several days, there seems to be no reason why such a balloon, at a height of four or five miles, could not pass from San Francisco to New York, or from the United States to Europe, since the motion of the upper clouds proves that the high atmosphere moves almost constantly with great velocity from the west to the east. The dirigible balloon has not been realized except in nearly calm weather, but the aeronaut can often reverse his direction by ascending or descending into a contrary wind to that in which he has been travelling. Frequently no clouds separate these opposing currents, which become apparent only when a balloon enters them.