The screw jack a is bolted to the plank c; at the other end of the plank is fixed the rack g, in which the toe of the strut advances as the serew b is elevated; the strut works in a joint in the follower k; the position of the strut when the screw is depressed is shown by the dotted lines. The object of this strut is to relieve the screw of the violent cross strain, to which the apparatus is subject when the engine or carriage is pulled over by the lever, which strain is entirely transferred to the strut, and the screw has merely to carry the load.
The operation of traversing the jack is as follows: - by hooking the link i upon the hook of the lever e, the toe of the lever being inserted into a ratch of the rack n of the lower plank, a man, bearing down the end of the lever, drags the apparatus and engine or carriage towards him with great facility: thesame lever is used to turn the screw and to produce the traverse motion. By this apparatus an engine of 16 tons weight has been replaced upon the rails in five minutes by the engineer and stoker alone; thus those delays which are the subject of so much annoyance and loss to railway proprietors and the public need not happen in future. The apparatus is exceedingly portable and cheap, and no train oughtto be allowed to go out without its being sent along with it: it may be carried either upon the tender, or upon some other place which may be selected for it.