The method of dropping the hooks is as follows:--Two levers, S, T, of different shapes are employed for each pair of hooks; these levers are centred on a rod, X. One of the levers, viz. T, is bent from the fulcrum to touch the bottom hook, and the lever S projects straight out from the fulcrum, and an upright needle O rests upon it, the top hook resting upon the upright needle. When the lever SY is lifted a little at Y it will drop the top hook, and when TY is lifted at Y it will drop the bottom hook.
In a 16 shaft dobby the parts shown in the diagram are duplicated sixteen times--that is, there are sixteen uprights MN, each with two hooks, sixteen levers SY, sixteen like TY, and sixteen of other parts. The levers SY and TY are operated by lags pegged so as to lift the staves to give the required pattern. These lags work round a cylinder or barrel, which is turned round the space of one lag every two picks intermittently. Each lag operates the hooks for two picks, one row of pegs operates the top hooks P, and the other row of pegs the bottom hooks Q. The method of pegging the lags will be understood from Fig. 92, where two lags are shown with the pegging for a two and two twill. Of course care must always be taken that the pegs are put opposite the proper levers, as when only a portion of the jacks are used, say eight, it is often preferred that the staves be connected to eight jacks in the middle of the machine.
Dobbies constructed with single jacks, as indicated in Fig. 91, are only suitable for narrow looms. Those constructed with double jacks are preferable for wider looms, as they not only keep the healds under better command, but they also move them in a perfectly vertical plane without the tendency to a slight side movement such as occurs when the healds are controlled by single jacks, in consequence of the ends of these describing an arc of a circle as they rise and fall. One of the best adaptations of double jacks to the Keighley type of dobby is that exemplified in the “Climax” dobby made by Lupton and Place, Burnley. This is represented in Fig. 93, in which A and B are complementary jack-levers operated from the same baulk-lever, J, controlled by hooks P and Q, to govern the same heald. The distinctive features of this dobby are the construction of the outer jack-lever A in one part, instead of two parts, and its attachment with the inner jack, B, by means of a link, C. This modification is a great improvement on double-jack dobbies in which the connections are made with streamer hooks or rods, or those in which the jacks are geared by means of toothed segments, as these increase the number of parts that are liable to wear and to get out of order.