[Illustration: _Detail of one type of sample-counting tube for carbon-14 work. Carbon-14 in benzene gas molecules is placed in the central cell and mixed with a fluid that scintillates, or emits light flashes, when exposed to radiation. Photomultiplier tubes convert the flashes to electric signals._] The radiocarbon-bearing gas is pumped into the sample counter through the filling tube, and all the counts resulting from its disintegrations are recorded electronically. The age of the sample is calculated from the NET COUNTING RATE (the sample counting rate minus the background); the lower the counting rate the higher the age. The upper limit of the age that can be measured is determined by the STATISTICAL ERROR (that is, by the measure of the instrument accuracy) in the net count. In very old samples this error may be great enough so that the calculated age of the sample may have little or no meaning. Carbon-14 Results OBJECTS DATED BY RADIOCARBON [Illustration: _Hair of an Egyptian woman. 5020 ± 290 years old._] [Illustration: _Linen wrapping from the Dead Sea Scroll containing the Book of Isaiah. 1917 ± 200 years old._] [Illustration: _Peruvian rope. 2632 ± 200 years old._] [Illustration: _Preglacial wood found in Ohio. More than 20,000 years old._] [Illustration: _Rope sandal found in an eastern Oregon cave. One of a pair of 300 pairs found in this cave. 9035 ± 325 years old._] Carbon-14 is by far the most widely used method of measuring geologic time. It has become the mainstay of archeology and geology for studies of events of the past 50,000 years or so, and also has wide applications in climatology, ecology, and geography. It would be difficult to pick out the most significant example of the use of this method, but one important contribution has been in study of the early inhabitants of North America. With the aid of ¹⁴C it has been possible to date human living sites from many points in the western United States. The first appearance of these sites, about 11,500 years ago, apparently coincided with the time when a land bridge was open from Asia to America over what is now the Bering Strait. An ice-free passage extended from this bridge through present-day Alaska and western Canada to the United States. This may have been the route taken by the first immigrants to America—a population of mammoth-hunters, who made the characteristic flint _Clovis_ arrow and spear points. [Illustration: _A Clovis arrow point chipped from flint by the earliest men on the American continent. The photograph is actual size._]