The polar ice cap rapidly recedes; colonies of honeybees collapse in alarming numbers; androgynous fish are detected in rivers and streams. These reports not only describe recent events, but also function as signs of an ominous and rapidly encroaching future. In this issue of Limn we focus on how this future makes its appearance in the present. Many of the threats we now find most alarming-climate change, environmental radiation, emerging disease, endocrine disrupters, toxic chemicals-are not immediately perceptible to human senses. We rely on non-human indicators, whether animals or detection devices, to alert us to their possible onset. Such indicators can be thought of as sentinels, or heralds of an approaching danger.

How new techniques of quantification shaped the New Deal and American democracy. When the Great Depression struck, the US government lacked tools to assess the situation; there was no reliable way to gauge the unemployment rate, the number of unemployed, or how many families had abandoned their farms to become migrants. In America by the Numbers, Emmanuel Didier examines the development in the 1930s of one such tool: representative sampling. Didier describes and analyzes the work of New Deal agricultural economists and statisticians who traveled from farm to farm, in search of information that would be useful for planning by farmers and government agencies. Didier shows that their methods were not just simple enumeration; these new techniques of quantification shaped the New Deal and American democracy even as the New Deal shaped the evolution of statistical surveys. Didier explains how statisticians had to become detectives and anthropologists, searching for elements that would help them portray America as a whole. Representative surveys were one of the most effective instruments for their task. He examines pre-Depression survey techniques; the invention of the random sampling method and the development of the Master Sample; and the application of random sampling by employment experts to develop the "Trial Census of Unemployment."