Evidence of Han mechanical engineering comes largely from the observational writings Confucian scholars. Professional artisan-engineers unfortunately did not leave behind detailed records of their work. Han scholars, who often had little or no expertise in mechanical engineering, sometimes provided insufficient information on the various technologies they described. Nevertheless, some Han literary sources provide crucial information. The belt drive, as described by Yang Xiong in 15 BCE, was first used to wind silk fibers onto the bobbins of weaver shuttles. This led to the development of other technologies during the Song Dynasty, such as the chain drive and the spinning wheel. The official and poet Sima Xiangru also hinted in his writing that the Han Chinese used a censer in the form of a gimbal, or a pivot support made of concentric rings which allowed the central gimbal to rotate on an axis while remaining vertically positioned. However, the first explicit mention of the gimbal used as an incense burner occurred around 180 CE when the artisan Ding Huan created his 'Perfume Burner'. It allowed burning incenses to be placed within the central gimbal, resulting in it remaining constantly level even when moved. Ding was proficient in other areas as well and had numerous other inventions. For the purpose of indoor air conditioning, he set up a large rotary fan powered manually which had rotating wheels that were 3 m in diameter. Ding also created the zoetrope lamp, which when lit, caused vanes placed on the top of the lamp to spin, which in turn rotated paper figure and other animals around the lamp. Zhang Heng wrote in the 2nd century CE that people could be entertained by theatrical plays of artificial fish and dragons. Later in the century the inventor Ma Jun built a theater of moving mechanical puppets that was powered by the rotation of a hidden waterwheel. From literary sources it is known that the collapsible umbrella was invented during Wang Mang's reign. It employed sliding levers and bendable joints that could be protracted and retracted. In addition, modern archaeology has led to the discovery of Han artwork portraying inventions which were otherwise absent in Han literary sources. This includes the crank handle. Han pottery tomb models of farmyards and gristmills possess the first known depictions of crank handles, which were used to operate the fans of winnowing machines. The machine separated chaff from grain, but was later used for silk-reeling, hemp-spinning, flour-sifting, and drawing water from a well using the windlass. The odometer cart was also created in order to measure distance traveled. Depicted in Han artwork by the 2nd century CE, detailed written descriptions were not offered until the 3rd century CE after the fall of the Easter Han. The wheels of the device rotated a set of gears which in turn forced mechanical figures to bang gongs and drums, alerting the travelers of the distance traveled, which was measured in li. It is also known that Han craftsmen made use of the sliding metal caliper to make minute measurements.