With materials as valuable as silk, wool, flax and cotton, and such of their products as are sold by weight, the actual weighing of the moisture they may happen to contain at the time of sale is of evident importance to both buyer and seller, when it is once understood what slight changes of circumstances can cause a loss or gain in weight involving thousands of dollars in many every-day transactions. When such materials are bought and sold on a conditioned weight basis, the standard dry weight of a shipment is obtained, upon which certain percentages of “regain” or added weight are allowed, varying with the hygroscopic capacity of each class of material. The author, after an investigation covering a period of over twenty years, has developed a set of laws governing regain in cotton and worsted, which are set forth in the paper with particulars regarding their derivation, and are accompanied by numerous charts and tables for facilitating calculations.