 Complete the following phrase:  It has been demonstrated that manned hot air balloons can be built using ancient materials. While there is no direct documentary or archaeological evidence that any manned or unmanned flights prior to those discussed below occurred using these methods, Ege notes an indirect report of evidence that the Chinese solved the problem of aerial navigation using balloons, hundreds of years before the 18th century. The Mongolian army studied Kongming lanterns from China and used them in the 