Chap. XII.

MACHINES, TOOLS, ETC.

1003

that described by the resistance, as the circumference of thescrew, is to the distance between two of the threads: insome of its applications, these powers are multiplied by onescrew working within another; the larger, turning roundwithin a fixed nut, lias a progressive motion, whilst thesmaller one is made to work within it.

For presses the application of the screw is very simple,although the friction which accompanies it materially affectsits power; but it is of service in maintaining the press inthe position in which the force has brought it; it is alwaysturned by a lever, without which the friction would renderit useless. The double screw may be used with consider-able advantage wherever a small extent of motion is required.

The screw of the printing press having to descend withgreat momentum, it is made more open; and in all suchcases the handle must be loaded.

For raising weights, the screw has been substituted forthe rack and pinion of the common jack, and for manypurposes it is far better adapted; the mason requires theuse of such a machine for the elevating the massive stoneshe has to work upon, and for ordinary purposes it is muchmore convenient than the lever.

Screw Jack _The block of wood has a hole bored through nearly its entire length for

the purpose of allowing the screw to move up and down without touching: at the top ofthe block is firmly fixed a nut, through which the screw passes when turned round. Theclaw fitted at the top and bottom of the screw has a round collar, which allows the screwto revolve without turning it: at the bottom of the block are four points to prevent itsslipping when used. The motion is given to the screw by a worm wheel fitted upon thesquare, the teeth of which are engaged by a worm on the axis of the winch ; when this isturned round, it causes the wheel to revolve by the action of the worm in its teeth, and asthe wheel is attached to the square part of the screw, it is compelled to turn with it,permitting the screw to move up and down at the same time.

Fig. 1510. screw jack. Fig. 1511. screw jack.

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Fig. 1509.

In another description of screw jack, not having so much friction, the screw does notturn round, but only rises and falls ; this is effected by the worm wheel being placed nearthe top of the block, which has a nut cut in its centre, through which the screw passes,and the nut is turned round instead of the screw. The axis of the worm is supported bysimilar iron plates to that already described, and is also turned by a winch.