It was China's longstanding priority that they protect themselves from nomads and they have accordingly returned to it. The growing limits on the Chinese navy would leave them vulnerable to foreign invasion by sea later on. As was inevitable, Westerners arrived on the Chinese east coast, primarily Jesuit missionaries which reached the mainland in 1582. They attempted to convert the Chinese people to Christianity by first converting the top of the social hierarchy and allowing the lower classes to subsequently convert. To further gain support, many Jesuits adopted Chinese dress, customs, and language. Some Chinese scholars were interested in certain Western teachings and especially in Western technology. By the 1580s, Jesuit scholars like Matteo Ricci and Adam Schall amazed the Chinese elite with technological advances such as European clocks, improved calendars and cannons, and the accurate prediction of eclipses. Although some the scholar-gentry converted, many were suspicious of the Westerners whom they called "barbarians" and even resented them for the embarrassment they received at the hand of Western correction. Nevertheless, a small group of Jesuit scholars remained at the court to impress the emperor and his advisors. Near the end of the 1500s, the extremely centralized government that gave so much power to the emperor had begun to fail as more incompetent rulers took the mantle. Along with these weak rulers came increasingly corrupt officials who took advantage of the decline. Once more the public projects fell into disrepair due to neglect by the bureaucracy and resulted in floods, drought, and famine that rocked the peasantry. The famine soon became so terrible that some peasants resorted to selling their children to slavery to save them from starvation, or to eating bark, the feces of geese, or other people. Many landlords abused the situation by building large estates where desperate farmers would work and be exploited. In turn, many of these farmers resorted to flight, banditry, and open rebellion. All of this corresponded with the usual dynastic decline of China seen before, as well as the growing foreign threats. In the mid-16th century, Japanese and ethnic Chinese pirates began to raid the southern coast, and neither the bureaucracy nor the military were able to stop them. The threat of the northern Manchu people also grew. The Manchu were an already large state north of China, when in the early 17th century a local leader named Nurhaci suddenly united them under the Eight Banners—armies that the opposing families were organized into. The Manchus adopted many Chinese customs, specifically taking after their bureaucracy. Nevertheless, the Manchus still remained a Chinese vassal.