



PlATK 



, ... 



599 



I & J. 



JACCATRA: See JAVA. 



JACK, is a word used by practical mechanics to de- 

 note any simple or trifling piece of mechanism. 



Jack* and Jack Sinker t, are parts of a stock ing- frame, 

 see CHAIN-WORK ; and lace machines are also provided 

 with parts called jacks. A harpsichord has a jack at 

 every key. Brewers Lave a large vat which they call 

 jack back. Masons employ a hand jack, screw jack, or 

 jack in a box. Carpenters use a jack plane. Jack towels 

 and roasting-jacks arc to be found in every kitchen. 



We stall liere give a description of the roasting-jack 

 and the hand- jack, both of which are complete machines 

 in themselves ; whereas mot of the other kinds of jacks 

 are only small parts of other machines. 



Hand Jack, or Jack in a Bar, is a simple and portable 

 mechanic*! power for assisting the hand in heaving up 

 or moving heavy bodies to a small distance, as for in- 

 stance, to raiie the end of a large block of stone or piece 

 of timber sufficiently to put rollers beneath to remove 

 it, to turn stone or tree over, or to put a chain 

 round it, to repair buildings, and tn lilt up an an- 

 gle of a wall to undermine the foundation. Many other 

 purpose* to which a machine of this description is ap- 

 plicable are too obvious. The simple lever, or wedge, 

 will, in many instances, perform all that is desired fruin 

 a hand jack ; but the lever will not retain the load at 

 the height to which it is raised, and the drawing of 

 wedge* is a tedious operation. The power of the com- 

 mon or simple hand jack, is obtained by a rack and pi- 

 nion. See Plate CCXC1X. Fig. 6. A block of wood about 

 2 feet (j inches long, 10 inches broad, and G inches wide, 

 is perforated with a square hole or mortice through it 

 lengthwise, for the reception of an iron rack B. This 

 rack is formed with a double claw or horn at its upper 

 end. A small pinion C is made to engage in the teeth 

 of the rack. The axis of the pinion is supported in 

 iron plates bolted to each side of the block, and one end 

 of the axis projects through the side plate, with a square 

 to receive a winch or handle, which, being turned 

 round, the pinion elevates the rack B in the mortice, 

 and raises the claw or bom up to the load to which 

 it is- applied. To prevent the weight of the load run- 

 ning the pinion hack, the handle is detained by a book 

 or link a fastened to the outside of the block. When 

 a greater power is required than the simple rack and 

 pinion are capable of exerting, a combination of wheel- 

 work is used, as shewn in the same Figure, where A A is 

 the block of wood, which in this case is made sufficiently 

 wide to contain the cog-wheel F, fixed to the pinion C, 

 which acts in thr teeth of the rack B. G is a second pi- 

 nion of four leave*, working in the wheel F ; and the axis 

 >:' this pinion projects through the side of the block for 

 the winch H to be fixed on it The block AA is made 

 in two halves, and the recess for the wheel F and pinion 

 (' is cut out in one of the halves ; the other, being 

 laid flat against it, supports the front pivot* of the 

 wheel and pinion*. The two halve* are bound toge- 

 ther by strong iron hoop* b, b, driven over the out&ide. 

 The rack has a claw N at its lower end, projecting out 

 tidewise through an opening or slit cut through, in the 

 This 



. 



front half of the block. 



claw can be introduced 



beneath a stone which lies nearly flat upon the ground, \_^^_* 

 and which consequently could not be acted upon by ""^ 

 the claw on the top of the rack. To prevent the rack 

 descending when it lias a load upon it, the small click a 

 drops into its teeth, but clears it in going up ; when it 

 is not required to detain the rack, this click can be turn- 

 ed out of the way sidewise. 



Fig. 7. is a screw-jack. The block of wood A A is Scr jack. 

 perforated nearly its whole length with a hole sufficient- 

 ly large to allow the screw B to move up and down 

 without touching. The screw passes through a nut 

 n, fixed firm into the top of the block A ; and if the 

 screw is turned round, it must rise up through the nut, 

 and elevate the claw F. This claw is fitted on the top 

 of the screw with a round collar, which allows the screw 

 to turn round without turning the claw ; and the claw 

 X, which projects through a groove or opening made in 

 the side of the block, is fitted to the screw with a 

 smaller collar. The bottom of the block has four short 

 points to prevent the machine- slipping when used upon 

 hard ground. To give motion to the screw, the lower 

 half of it is formed into a square, and a worm-wheel C is 

 fitted upon the square. The teeth of this wheel are en- 

 gaged by a worm on the axis of the winch H, and plates 

 of iron a, b, are bolted on each side of the block, near 

 the middle of its height, to carry the ends of the axis of 

 the winch, and of the worm which is concealed behind 

 the worm-wheel C. When the winch is turned round, 

 it causes the wheel C to revolve by the action of the 

 worm in its teeth ; and as the wheel is fitted on the 

 square part of the screw, it compels it to turn with it, 

 but at the same time allows the screw to move up and 

 down. 



The friction of this kind of screw jack is very consi- Another 

 derable ; nor is its strength so great as the screw-jack *rew ja^ 

 shewn in Figs. 8. and 9. In this the screw is not made * '8*' ** ' 

 to turn round, but only to rise and fall ; and the worm- 

 wheel C, being placet! very near the top of the block A, 

 has a nut, for the screw B to pass through, cut in its 

 centre. By this means the nut is turned round instead 

 of the screw, and it requires no collar either at the top 

 or bottom of the screw. The axis of the worm, like the 

 former screw-jack, is supported by two iron plates a, b, 

 and the end which projects through the plate a is made 

 square to apply the winch H. The worm-wheel is 

 shewn dotted in Fig. 9. The circular part D is a part 

 of the wheel C, and forms a collar for the wheel, and 

 gives also a greater depth of nut for the screw to work 

 in. 



Fig. 10. shews a jack on the hydrostatic principle dis- Bnnuh'* 

 covered by Pascal, and applied to practice by the late ! ) y^ ow ti 

 Mr Bramah to presses, crane*, and various other pur- ^ 10< 

 pose*. It is the most powerful of any machine what- 

 ever, and very commodious. A is a large blcck of 

 wood, bolted together in two halves around a cast-iron 

 cylinder, which resembles a mortar or cannon. It is 

 doted at its lower end, and furnished with a piston B, 

 which is turned truly cylindrical, and embraced by a 

 collar of leather in the head of the cylinder a ; which 

 will prevent any water from leaking out of the cylinder 

 by the side of the piston ; a small copper pipe b is con- 



