But to the extent that weather cannot be predicted it may be influenced. If small causes may have big effects then even the puny means available to man may change the weather—provided we know how and where to apply the lever. First we shall have to acquire a better understanding of the weather-science of meteorology. Then we shall have to look for the appropriate trigger mechanism. This may be a cloud of dust of the right kind—or else a chemical—or perhaps a great number of radioactive particles. In one way or another atomic explosions may be used as the trigger but the trigger will not be effective until and unless the rest of the machinery is understood. Of course atomic explosions cannot be used in really significant numbers unless we learn how to avoid those radioactive by-products which are really dangerous. Fortunately the use of nuclear fusion, best known from the hydrogen bomb, makes it possible to regulate the kind of radioactivity one obtains. We may make only such kinds of activity which decay before they have a chance to get into the human body. Experience has proved that to talk about weather is not dangerous. To do something about the weather will be more risky. Shall the weather become a ward of the government? Shall we have Republican Rainstorms and Democratic Droughts? In this way we shall certainly lose the last safe topic of conversation. In the narrower confines of Europe where sovereign nation is a few hours from sovereign nation (as the wind blows) the situation will be much more serious. But even the whole planet may prove too small for fiercely conflicting interests when more knowing fingers are placed on more sensitive triggers. To govern the weather can be most useful. It could give ample livelihood to all the people of the earth and to many more billions. Such endeavor is surely good and it would appear peaceful. But in this case as in many other cases knowledge will lead to power and power will lead to disaster if it is not tempered by wisdom. Yet this knowledge or some similarly dangerous knowledge will come to us in our lifetimes. Nuclear explosions do not stand alone as a potential source of mischief. CHAPTER XVII Safety of Nuclear Reactors At the beginning of the scientific and industrial revolution two old ambitions were found to be impossible dreams. One was the transmutation of elements, the other the machine of perpetual motion. Modern nuclear physicists had to retract one of these statements: elements can be transmuted. But the product is expensive, for the time being much more expensive than gold. The perpetual motion machine remains impossible in principle but the problem may be considered solved in practice. It can be proved, of course, that a machine can do useful work only if it burns up some fuel. But the price of fuel is quite often less than the cost to operate and maintain the machine.