1562

THEORY AND PRACTICE OF ENGINEERING.

Book II.

not permit us to detail the construction of many portions of those machines, the principlesof which have been already described: we can only allude to the movements of a few ofthose most generally in use.

In forming a screw the hardest wood should be selected, as that of the box tree, orany other of equal strength, which is not likely to warp or get out of form. As thepower of the screw depends upon the correctness with which the thread is turned, theprimitive cylinder, which the inclined plane is to surround, should be worked very accu-rately before it is offered to the lathe or cutting machine.

When the screw is mounted to serve the purposes of a press, the standards which confineits action must be braced and firmly held together; the braces at the sides may be made toserve as supports and ties.

Where two screws are requiredto lift the same plane, great at-tention must be paid to the cuttingof the spiral threads around them,or they will work unequally, and

O

Fig. 2911.

throw greater stress upon one thread than the other.

The framework upon which the spindles of wheels are mounted, as well as the axles,should be wrought with the greatest nicety, and the staves of the trundles and the cogs ofthe wheels made either of hornbeam wood or lignum vitae: in mounting a trundle Per-ronet used two triangularly framed stands, into the heads of the upright posts of whichthe gudgeons worked.

Fig. 2915.

Fig. 2913.

When a beam is to be lifted perpendicularly, the wipersrequire to be fixed firmly, otherwise the cams on theturning wheels, which produce its motion, and occasion itto rise and fall successively, will not act with that precisionwhich is required for stamping or pounding, to which thismotion is particularly applicable. After passing the wipersthrough a mortise made to receive them, they requirefirmly wedging from the opposite side.

Tread Wheels are of very simple construction, andserviceable in moving pumps or other machines; theyhave a long iron shaft, with four or six wheels about 40inches in diameter, 24 spurs or starts, like those of an un-dershot water-wheel, being fixed upon it; these spurs are8 or 9 inches in length, with footboards attached bystrong screws : the useful effect of a tread mill has beencalculated upon the supposition that a man can lift his ownweight 12,000 feet high in a day; supposing him to weigh150 pounds, the total weight lifted would be equal to1,800,000 pounds raised a foot in the course of a day’swork of 8 hours ; walking wheels are variously made, andthere are several with footboards attached to the leversthat move the spindle round.

mm

Fig. 2916. wipers c