6There is a machine, on the principle of the screw, which raises water with considerable power, but not so high as the wheel. It is contrived as follows. A beam is procured whose thickness, in inches, is equal to its length in feet; this is rounded. Its ends, circular, are then divided by compasses, on their circumference, into four or eight parts, by diameters drawn thereon. These lines must be so drawn, that when the beam is placed in an horizontal direction, they may respectively and horizontally correspond with each other. The whole length of the beam must be divided into spaces equal to one eighth part of the circumference thereof. Thus the circular and longitudinal divisions will be equal, and the latter intersecting lines drawn from one end to the other, will be marked by points.
2These lines being accurately drawn, a small flexible ruler of willow or withy, smeared with liquid pitch, is attached at the first point of intersection, and made to pass obliquely through the remaining intersections of the longitudinal and circular divisions; whence progressing and winding through each point of intersection it arrives and stops in the same line from which it started, receding from the first to the eighth point, to which it was at first attached. In this manner, as it progresses through the eight points of the circumference, so it proceeds to the eighth point lengthwise. Thus, also, fastening similar rules obliquely through the circumferential and longitudinal intersections, they will form eight channels round the shaft, in the form of a screw.
3To these rules or slips others are attached, also smeared with liquid pitch, and to these still others, till the thickness of the whole be equal to one eighth part of the length. On the slips or rules planks are fastened all round, saturated with pitch, and bound with iron hoops, that the water may not injure them. The ends of the shaft are also strengthened with iron nails and hoops, and have iron pivots inserted into them. On the right and left of the screw are beams, with a cross piece at top and bottom, each of which is provided with an iron gudgeon, for the pivots of the shaft to turn in, and then, by the treading of men, the screw is made to revolve.
4The inclination at which the screw is to be worked, is equal to that of the right angled triangle of Pythagoras: that is, if the length be divided into five parts, three of these will give the height that the head is to be raised; thus four parts will be the perpendicular to the lower mouth. The method of constructing it may be seen in the diagram at the end of the book. I have now described, as accurately as possible, the engines which are made of wood, for raising water, the manner of constructing them, and the powers that are applied to put them in motion, together with the great advantages to be derived from the use of them.