jack when engaged by the slurcock as the number of sinkers in the process of being advanced at any time need not be changed at high speed the extent of the slurcock lift can be halved compared with conventional slurcocks engaging a single jack at a leverage of 3 it has been found that at a leverage of approximately 3 smooth operation may be achieved whilst measuring a stable amount of yarn that is to say the amount of yarn drawn which governs the stitch length is substantially unaffected by the speed of slurcock traverse which may be low when operating manually and high when operating under power at leverages of 4 operation is smooth under power at high speed but the amount of yarn measured may not always be the same the first and second jacks may be accommodated in a compact manner being behind and below a sinker bar and above a head rail by making the first and second jacks of similar dimensions and arranging them substantially parallel and upright advantageously the second pivot is substantially at the same level as the slur bar and the first pivot is below the level thus both pivots are located close to one another and the head rail the jacks may be optimally designed and mounted for their respective functions the first jacks being arranged to absorb the sideways force exerted by the slurcock without risk of misalignment between second jacks and associated sinkers the first and second jacks thus may have each a reduced cross section having regard to conventional jacks preferably the jacks are mounted between jack walls arranged so as to expose the tips of the second jacks and a rear edge part.