[147] According to the National Bureau of Economic Research's definition, business cycles are wavelike fluctuations in aggregate economic activity of one to twelve years in duration. What distinguishes the cycle from random disturbances to trade is its recurrent nature and the systematic relationship of fluctuations in national product to the level of activity in different sectors .... it is imposibble to to date the birth of the cycle. But whether the business cycle is viewed as a specific characteristic of industrial economies or a universal concomitant of the market mechanism, there would be general agreement that it was fundamentally transformed by the process of industrialization. Hence, this note considers only the business cycle in industrial Britain, from the 1830s onward. .... theories of British business cycles experience recurrent waves of popularity. Perhaps the most durable of these explanations is that fluctuations in export demand provided the impetus for the trade cycle. This explanation has been applied equally to both halves of the XIXth century: for the period prior to 1850, Gayer, Rostow and Schwartz argue that business cycles in Britain 'were in large measure a function of fluctuations in the export demand for textiles;' similarly Ford argues that '... the proximate cause of cyclical fluctuations in the United Kingdorm from 1860 to 1914 lay in the behavior of exports of goods and services, aided sometimes by home investments...'. Even authors primarily concerned with other factors concede that British business cycles '...tended to shape in function of foreign business fluctuations.' (Schumpeter).