Jack in Mechanics, a portable machine for raising great weights through a small space. It consists of a rack and pinion inclosed within a strong wooden case, and the power is applied by means of a winch or handle fixed upon the axis of the pinion; the upper end of the rack is formed into two horns, to take the better hold of the article to be elevated; and from the lower end, two prongs project laterally through a longitudinal groove in the case, which are used upon occasions when there is not room to introduce the jack beneath the load. To prevent the labourers being overpowered, there is a ratchet-wheel and pall on the axis of the pinion.