This kind of system can be represented crudely by an old-fashioned hourglass, as shown in the figure, which has the parameters of these equations marked. (Keep in mind, however, that this is only a gross analogy. Nuclear clocks run at logarithmically decreasing rates, but the speed of a good hourglass is roughly constant.) [Illustration: _An hourglass illustrates an ideal closed system. Nothing is added and nothing is removed—the sand just runs from the top bulb to the bottom._] Remember that the decaying nucleus does not disappear. It changes into another nucleus, and this new nucleus forms an atom that may be captured and held fixed by natural processes. The decayed nuclei are thus collected, so that here we have the bottom chamber of the hourglass. But sometimes we need only the top chamber of an hourglass. THE CARBON-14 CLOCK Carbon-14 decay is the best example of a top-only hourglass. Carbon-14 is constantly being produced in the upper atmosphere from atoms of nitrogen-14 being struck by neutrons that had their origin in COSMIC RAYS. The reaction is written: ¹⁴N + neutron → ¹⁴C + proton Radioactive decay then follows, with a half-life of 5800 years[9] for the ¹⁴C. ¹⁴C → ¹⁴N + electron (BETA PARTICLE) The radiocarbon emits an electron and changes back into nitrogen. As far as anyone can tell, ¹⁴C was produced at a constant rate above the earth for at least 50,000 years before the first atomic bomb was exploded. In other words, the ¹⁴C cycle is like an hourglass in which the sand in the upper part is replenished as fast as it runs out through the hole in the waist. A process of this sort, where production equals decay, is called a SECULAR EQUILIBRIUM. The newly produced ¹⁴C soon is evenly mixed with the carbon dioxide in the air, is taken up by all living plants, and then finds its way into all living animals. In effect, all carbon in living organisms contains a constant proportion of ¹⁴C. If any of this carbon is taken out of circulation—when a tree branch is broken off, for instance, or when a shellfish dies in the ocean—no more new ¹⁴C is added to that particular system, but the old ¹⁴C continues to run out. In effect it now starts measuring time as an hourglass should. [Illustration: _To illustrate secular equilibrium, one must imagine an hourglass in which the sand in the top bulb is continuously replenished—as fast as it runs out through the hole in the waist and disappears._]