On the practical side, the food-preserving effect of low temperatures had long been recognised. Natural ice was vigorously traded in the early nineteenth century, but it was inevitably in short supply, especially in Australia. During the eighteenth and nineteenth centuries, there was considerable commercial incentive to develop ever more effective refrigerators thanks to the expansion of agriculture in the Americas, Australia, and New Zealand and rapid urbanization in Western Europe. From the 1830s onward, refrigerators relied on the expansion of compressed air or the evaporation of a volatile liquid; evaporation became the basis of all modern refrigerator designs. Long-distance shipping of perishable foods, such as meat, boomed in the late 1800s.