The Qin envoy Gan Zong reported from Xicheng that:
In foreign lands there are people who can summon deities by incantations. Approaching a river, adopting the ‘Yu walk’* and blowing, they cause a dragon to appear. On first emerging, it would be several tens of zhang** in length, but when the scholar blew again it would shrink, reaching only several cun,*** at which he would put it in a flask (rather teapot-shaped), feeding it with a little water. Foreign realms often suffer from severe drought, and when the scholar hears of a place with such as drought, he will send the dragon, taking it out to sell. One dragon is worth several catties of gold. The entire realm will gather to see it, and, once in place, he will open the pot and release the dragon. Placed in a deep pool, once the ‘Yu walk’* and blowing has been performed, it will grow to several zhang. In a moment rain will gather from the four directions.
From Baopuzi.
* The ‘Yu walk’ yubu 禹步 here refers to an imitation of the legendary Emperor Yu’s limp, reportedly employed as a magical technique. My thanks to the ever-helpful Ofer Waldman for bringing this to my attention and vastly improving this translation.
** A zhang 丈 is usually reckoned at about 3.3m in length.
*** A cun 寸 is usually reckoned at about 3.3cm in length.
Li Fang 李昉, et al., Taiping guangji 太平廣記 (Extensive Gleanings from the Era of Great Harmony), 10 vols (Beijing: Zhonghua shuju, 1961), ix, 418.3402:
甘宗
秦使者甘宗所奏西城事云。外國方土能神呪者。臨川禹步吹氣。龍卽浮出。初出。乃長數十丈。方士吹之。一吹則龍輒一縮。至長數寸。乃取置壺中。以少水養之。外國常苦旱災。於是方士聞有(有原作而。據明鈔本丶陳校本改。)旱處。便賚龍往。出賣之。一龍直金數十觔。擧國會歛以顧之。直畢。乃發壺出龍。置淵中。復禹步吹之。長數十丈。須臾雨四集矣。
出抱朴子