The friction of this kind of screw-jack is very considera ble; nor is its strength so great as the screw-jack shewn in Figs. S. and 9. In this the screw is not made to turn round, but only to rise and fall; and the worm-wheel C, being placed 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 io.
Fig. 10. shews a jack on the hydrostatic principle dis covered by Pascal, and applied to practice by the late Mr Bramah to presses, cranes, and various other purposes. It is the most powerful of any machine whatever, and very commodious. A is a large block of wood, bolted together in two halves around a cast-iron cylinder, which resembles a mortar or cannon. It is closed at its lower end, and furnished with a piston B, which is turned trt,ly cylindrical, and embraced by a collar of leather in the head of the cy linder a ; which will prevent any water from leaking out of the cylinder by the side of the piston; a small copper pipe b is connected with the cylinder near its top ; the other extremi ty of this pipe is carried to a small injecting pump C, stand ing in the cistern R, and actuated by the lever H. At each stroke of the pump a small quantity of water is forced or injected into the large cylinder, and this causes the piston B to rise through the collar of leather a very small quanti ty each time. This quantity will bear the same proportion to the motion given to the piston of the pump C, as the urea of that piston bears to that of the large piston B. The force applied to the small pump, and that exerted by the great cylinder, will be in the inverse proportion of these areas.* The pump rod is confined to a perpendicular motion by an iron frame P, through which it passes.
The top or head of the large cylinder is made larger than the lower part, as shewn by the dotted lines, and the shoul der which rests on the top of the block to sustain the cylin der. When the weight or burden is to be lowered down, it is done by opening a discharge valve in the pump, the handle of which is seen at : this allows the water to return by the small copper pipe b, and escape from the cylinder a into the pump cistern R. When the power of the lever
H is not found sufficient to perform its work, it can be augmented, by taking out the centre pin r, which forms the fulcrum of the lever, and inserting it into another hole which is nearer the centre of the pump.
Roasting Jack, is a machine for turning round meat when placed before the fire, so as to expose every part of the surface to the heat. These jacks are constructed it, dif ferent ways; and are either put in motion by dogs, by the smoke which ascends the chimney, by a weight, or by a spring.
When a dog is employed, the spit on which the meat is placed is mounted before the fire, and put in motion by a wheel, in which the dog walks in the same manner as men turn the walking wheel crane; this kind of jack is now out of use.
A smoke jack is represented in Fig. 5. where AA is the brickwork of the chimney, which is contracted to a circular figure at AA; B is a strong iron bar, placed upon an arch of brickwork over the lire place, to form a support for the wheel-work of the jack; C is a horizontal wheel placed in the circular funnel AA ; it is made of iron plate, with a number of vanes radiating from the centre, each being placed at an angle of about 30° to the plane of the wheel; the heated air which ascends the chimney will strike the vanes and turn the wheel; the lower end of the spindle D, on which the wheel is fixed, has a pinion E fixed on it, and turns a contrate or crown wheel F on the horizontal spindle G; on the other end of this spindle is a pulley H, for an end less chain, which descends to the spit on which the meat is placed; the pulley pinned on the end of the spit is laid in the loop formed by the endless chain, so that this chain at the same time suspends one end of the spit, and gives mo tion to it; the upper pivot of the spindle D is supported in an iron bracket m, projecting from the inside of the chim ney, and the lower pivot rests on the top of a standard n, which also carries one end of the horizontal spindle G; when the chimney requires to be swept, the wheel C is lifted out of its centre, to allow a boy to pass; these jacks are always in motion when the fire is kept up.