The C^{14} method of dating, an American device discovered and perfected by Professor W. F. Libby and his associates at the University of Chicago Institute of Nuclear Studies, is sufficiently new to deserve a word of explanation here. All living matter has a uniform radioactivity associated with its carbon content. The supply of the radioactive isotope C^{14} ceases when living matter, wood, foliage, etc., dies. Scientists can calculate the time elapsed since death by counting the residual radioactivity of C^{14} in the organic specimen, since the rate of decay can be described by specifying how long it takes for half the number of atoms in a given sample to disintegrate. For C^{14} this period, called its “half-life,” is 5700 years. If the present assay of a specimen of organic matter, for instance, is 12.5 C^{14} explosions per minute per gram of carbon, an ancient organic sample assaying at 6.25 would be 5700 years old (the half-life of C^{14}). Checking with samples of known date has proved the method accurate within 200 years either way. For most classical objects found in association with organic matter this is valueless, since a trained archaeologist can date a pot, an inscription, or an architectural block by eye within fifty years or less. But the method is invaluable for making more precise the great sweeps of time in prehistory. Thus the lowest C^{14} date for Su Nuraxi, 1070 B.C., would take it almost into the Iron Age in Italy; at this date culture on the mainland was much more primitive. Lilliu calls the period of the four added towers Lower Nuragic I, and dates it 800–750 B.C. These smaller towers contain each a single cell with two rows of loopholes. They are guard-posts, and are equipped with speaking-tubes for the guards use when challenging. In the next period, Upper Nuragic I, dated by Lilliu 750–500 B.C., the earth having subsided, the four towers and the walls were reinforced. The ground-level entrance was blocked, and replaced by a new entrance twenty-one feet higher, accessible only by ladder. Battlements now replaced the loopholes. Stone balls found in the excavations were apparently the projectiles hurled from these battlements. From a watchtower added to the central _nuraghe_ come conch shells, perhaps intended to be sounded like trumpets.