138

The Screw.

[Book I.

This machine has been constructed in a variety of ways. Sometimesby winding, in the manner of a screw, one or more flexible tubes

(generally of lead or strong leath-er) round a cylinder of wood oriron. This cylinder is sustainedby gudgeons in such a position,that at whatever angle with thehorizon it is used, the plane ofthe helix must always be inclinedto its axis at a greater angle ; oth-erwise no water could be raisedby it any more than by turningit in the wrong direction. Thelower end being immersed in wa-ter, the liquid enters the tube andis gradually raised by each revo-lution until it is discharged above.These machines are commonlyused at an inclination to the hori-zon of about 45°, although theysometimes are placed at 60°. Seethe figure.

No. 58. Screw. Instead of tubes wound round

a cylinder, large grooves were sometimes formed in the latter and cover-ed by boards or sheets of metal, closely nailed to the surfaces betweenthe grooves—so that the latter might be considered as tubes sunk intothe cylinder, instead of being folded round its exterior.

No. 59. Roman Screw.

Another mode was to make the threads of plank, arranged as a helixround a solid cylinder, which was fitted with journals, and made torevolve in a fixed, hollow cylinder of the same length; the edges or ex-tremities of the threads rubbing against the sides of the latter, and con-sequently producing the same effect as No. 58. This modification of thecochleon is known as the German Snail. It has this advantage, that itmay be worked in an open channel, or half a cylinder instead of a wholeone, since it is only the lower half of the latter, that is essential to thethe Operation of raising water. Machines of this kind of large dimen-sions have long been employed by the Dutch, and are generally dnven