An _aerostat_ (non-dirigible balloon), unless anchored, drifts at the speed of the wind. To the occupants, it seems to stand still, while the surface of the earth below appears to move in a direction opposite to that of the wind. In the sketch, if the independent velocity of a _dirigible_ balloon be _PB_, the wind velocity _PV_, then the actual course pursued is _PR_, although the balloon always points in the direction _PB_, as shown at 1 and 2. If the speed of the wind exceed that of the balloon, there will be some directions in which the latter cannot progress. Thus, let _PV_ be the wind velocity and _TV_ the independent speed of the balloon. The tangents _PX_, _PX´_, include the whole "field of action" possible. The wind direction may change during flight, so that the initial objective point may become unattainable, or an initially unattainable point may be brought within the field. The present need is to increase independent speeds from thirty or forty to fifty or sixty miles per hour, so that the balloon will be truly dirigible (even if at low effectiveness) during practically the whole year. [Illustration: INFLUENCE OF WIND ON POSSIBLE COURSE] Suppose a dirigible to start on a trip from New York to Albany, 150 miles away. Let the wind be a twenty-five mile breeze from the southwest. The wind alone tends to carry the balloon from New York to the point _d_ in four hours. If the balloon meanwhile be headed due west, it would need an independent velocity of its own having the same ratio to that of the wind as that of _de_ to _fd_, or about seventeen and one-half miles per hour. Suppose its independent speed to be only twelve and one-half miles; then after four hours it will be at the position _b_, assuming it to have been continually headed due west, as indicated at _a_. It will have traveled northward the distance _fe_, apparently about sixty-nine miles. [Illustration: COUNT ZEPPELIN] After this four hours of flight, the wind suddenly changes to south-southwest. It now tends to carry the balloon to _g_ in the next four hours. Meanwhile the balloon, heading west, overcomes the easterly drift, and the balloon actually lands at _c_. Unless there is some further favorable shift of the wind it cannot reach Albany. If, during the second four hours, its independent speed could have been increased to about fifteen and a half miles it would have just made it. The actual course has been _fbc_: a drifting balloon would have followed the course _fdh_, _dh_ being a course parallel to _bg_. GAS AND BALLAST A cubical block of wood measuring twelve inches on a side floats on water because it is lighter than water; it weighs, if yellow pine, thirty-eight pounds, whereas the same volume of water weighs about sixty-two pounds. Any substance weighing more than sixty-two pounds to the cubic foot would sink in water. [Illustration: BUOYANT POWER OF WOOD]