【正文】 s rock art is constantly being revised, and earlier datings have been proposed as the result of new discoveries. {27} █{/27}Currently, eliable scientific evidence dates the earliest creation of art on rock surfaces in Australia to somewhere between 30,000 and 50,000 years ago. {27}█ {/27}This in itself is an almost inprehensible span of generations, and one that makes Australia39。 that is about a third of the Dutch population. [2]Importing the grain, which would have been expensive and time consuming for the Dutch to have produced themselves, kept the price of grain low and thus stimulated individual demand for other foodstuffs and consumer goods.[/2] Apart from this, being able to give up laborintensive grain production freed both the land and the workforce far more productive agricultural divisions. The peasants specialized in livestock husbandry and dairy farming as well as in cultivating industrial crops and fodder crops: flax, madder, and rape were grown, as were tobacco, hops, and turning. These products were bought mostly by 滿分網(wǎng)（ ） urban businesses. There was also a demand among urban consumers far dairy products such as butter and cheese, which, in the sixteenth century, had bee more expensive than grain. The high prices encouraged the peasants to improve their animal husbandry techniques。 and if a house were perfectly insulated, one adult could also produce more than enough heat to warm it. Therefore, even without any industrial production of heat, an urban area tends to be warmer than the countryside that surrounds it. The burning of fuel, such as by cars, is not the only source of this increased heat. Two other factors contribute to the higher overall temperature in cities. The first is the heat capacity of the materials that constitute the city, which is typically dominated by concrete and asphalt. During the day, heat from the Sun can be conducted into these materials have a significantly lower heat capacity because a vegetative blanket prevents heat from easily flowing into and out of the ground. The second factor is that radiant heat ing into the city from the Sun is trapped in two ways: (1) by a continuing series of reflection among the numerous vertical surfaces that buildings present and (2) by the dust dome, the cloud like layer of polluted air that most cities produce. Shortwave radiation from the Sun passes through the pollution dome more easily than outgoing long wave radiation does。 the latter is absorbed by the gaseous pollutants of the dome and reradiated back to the urban surface. Cities, then, are warmer than the surrounding rural areas, and together they produce a phenomenon known as the urban heat island. Heat islands develop best under particular conditions associated with light winds, but they can form almost any time. {13}█ {/13}The precise [7]configuration[/7] of a heat island depends on several factors. {13}█ {/13}For example, the wind can make a heat island stretch in the direction it blows. {13}█ {/13}When a heat island is well developed, variations can be extreme。 for example, they began feeding their animals indoors in order to raise the milk yield of their cows. In addition to dairy farming and cultivating industrial crops, a third sector of the Dutch economy reflected the way in which agriculture was being modernizedhorticulture. {13} █ {/13}In the sixteenth century, fruit and vegetables were to be found only in gardens belonging to wealthy people. {13}█ {/13}This changed in the early part of the seventeenth century when horticulture became accepted as an agricultural sector. {13}█ {/13}Whole villages began to cultivate fruit and vegetables. {13}█ {/13}The produce was then transported by water to markets in the cities, where the [5]consumption[/5] of fruit and vegetables was no longer restricted to the wealthy. As the demand for agricultural produce from bath consumers and industry increased, agricultural land became more valuable and people tried to work the available land more intensively and to reclaim more land from wetlands and lakes. In order to increase production on existing land, the peasants made more use of crop rotation and, in particular, began to apply animal waste to the soil regularly, rather than leaping the fertilization process up to the grazing livestock. For the first time industrial waste, such as ash from the soapboilers, was collected in and the cities and sold in the country as artificial fertilizer. The increased yield and price of land justified reclaiming and draining even more land. The Dutch battle against the sea [10]legendary[/10] Nvorderkwaroer in Holland, with its numerous lakes and stretches of water, was particularly suitable for land reclamation and one of the biggest projects undertaken there was the draining of the Beemster lake which began in 1608. The richest merchants in Amsterdam contributed money to reclaim a good 7,100 hectares of land. Fortythree windmills powered the drainage pumps so that [8]they[/8] were able to lease the reclamation to farmers as early as 1612, with the investors receiving annual leasing payments at an interest rate of 17 percent. Land reclamation continued, and between 1590 and 1665 almost 100,000 hectares were reclaimed from the wetland areas of Holland, Zeeland, and Friesland. However, land reclamation decreased significantly after the middle of the seventeenth century because the price of agricultural products began to fall, making land reclamation far less profitable in the second part of the century. Dutch agriculture was, finally affected by the general agricultural crisis in Europe during the last two decades of the seventeenth century. However, what is [11]astonishing[/11] about this is not that Dutch agriculture was affected by critical phenomena such as a decrease in sales and produ