After unearthing an ancient crossbow device from a house's garden in Haichow, Jiangsu, Shen discovered that the cross-wire grid sighting device, marked in graduated measurements on the stock, could be used to calculate the height of a distant mountain in the same way that mathematicians could apply right-angle triangles to measure height. Needham asserts Shen had discovered the survey device known as Jacob's staff, which was not described elsewhere until the Provençal Jewish mathematician Levi ben Gerson (1288–1344) wrote of it in 1321. Shen wrote that while viewing the whole of a mountain, the distance on the instrument was long, but while viewing a small part of the mountainside the distance was short due to the device's cross piece that had to be pushed further away from the observer's eye, with the graduation starting on the further end. He wrote that if one placed an arrow on the device and looked past its end, the degree of the mountain could be measured and thus its height could be calculated.