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ferential and longitudinal intersections, they will form 
eight channels round the shaft, in the form of a screw. 
To these rules or slips others are attached, also smeared 
with liquid pitch, and to these still others, till the thick- 
ness 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. The in¬ 
clination 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 dia¬ 
gram 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.