With the second configuration enclosed in a lead box, the results showed that some cosmic rays interact in lead to produce multiple secondary particles. In an extension of the second experiment, he measured the rate of triple coincidences as a function of the amount of the lead above the counters. A plot of this rate against thickness, which came to be known as the Rossi curve, showed a rapid rise as the lead layer was increased, followed by a slow decline. These experiments showed that ground-level cosmic rays consist of two components: a "soft" component which is capable of prolific generation of multiple particle events, and a "hard" component which is capable of traversing great thicknesses of lead. At the time, the physical nature of both was a mystery, for they did not yet fit into the growing body of knowledge about nuclear and particle physics.