In ascertaining the contents of land, it is usual in measuring on a sloping surface to make allowance for the difference between the sloping length and the true horizontal distance, the latter being the length for buying or selling and for plotting on paper. There are various instruments and tables for giving this allowance, or it may be calculated thus: A fall of oft. vertical in a length of 80 ft. on the slope would give a horizontal distance of = 79'8ift. A fall of 10ft. in 180 ft. would give a horizontal distance of = 179.72 ft. Usually, the measurements are taken with a chain of 06 ft., and an allowance per chain, according to the slope in degrees, is made by pulling the chain forward 1/4 link, or whatever the requisite allowance may be, beyond the arrow, and then shifting the arrow forward.