[Illustration: A. A fix determined in two dimensions. The lines of sight of the runners on first and third intersect at x. B. A fix determined in three dimensions. The lines of sight of the runners on the first and third intersect at x.] Now, let us move into the _third_ dimension, since a fireball’s path through the atmosphere lies in space, not in the “flat” plane of the earth’s surface. Returning to our baseball diamond, let us suppose that a helicopter with an enterprising photographer aboard hovers over the centerfield bleachers so that he can take pictures of the record crowd. While the umpire is dusting off home plate, the two runners on first and third simultaneously sneak a look to see what the helicopter is doing. Their lines of sight now intersect at the helicopter and fix its position _in space_. Similarly, the location of a fireball path in space is determined by the fixing of certain points on the luminous streak seen in the sky. Instead of using only two intersecting lines of sight (those of the runners on first and third in our analogy), scientists investigating a meteorite fall try to collect as many different lines of sight as possible from people in the region above which the fireball streaked. The more commonly determined points are those of the fireball’s appearance and disappearance and those where “explosions” took place. These points are generally located by use of the method we have described in some detail above, the so-called _intersecting-lines-of-sight_ method. The most important point on a fireball path is the point of disappearance. The most valuable single piece of information you can supply about a meteorite fall is as accurate an answer as possible to the question: In what compass direction were you looking when you _last_ saw the fireball? This question has often been twisted around in newspaper and radio accounts into the meaningless question: In what direction was the fireball going when you saw it? One person cannot give the answer to the second question because from a single station it is impossible to determine the _true_ direction of motion of an object seen in the sky. One person can report only an _apparent_ direction of motion, which is of little or no value in locating the last point on the luminous path, generally referred to as the “end-point.” Therefore, though you cannot by yourself determine the actual direction in which a fireball is _moving_, you can report the direction in which you were _looking_ when you last saw the fireball, that is, due south, southwest, northeast, etc.