62 
MODIFICATION BY THE AUTHOR. 
The chief defect in the arrangement, however, is the making the blades plane 
surfaces instead of sections of screws. The result will either be excessive dis 
placement at the outer portions, or a direct opposition to the vessel’s progress 
at the parts near the centre. 
MODIFICATION BY THE AUTHOR. 
By reference to page 32 it will be seen that the parts of the screw near the 
centre expend the greatest portion of their effort in turning the water round 
without aiding considerably in the propulsion of the vessel. The object of several 
of the inventions we have described has partly been to obviate this defect. The evil 
may be in a great degree remedied without departing considerably from the form 
of the screw of the Archimedes. To demonstrate this, let us suppose the inner 
portion of that screw to be removed, and the outer part to be merely attached by 
arms radiating from the axis to the leading and after end of the screw, and that 
to the leading arms a number of cords or flexible lines be affixed. Under such 
circumstances, if the screw be made to propel a vessel at its maximum velocity, 
these cords or flexible lines will generate helices in their progress through the 
water. The pitch of these helices, however, will be less than the pitch of the 
screw, inasmuch as they will coincide with the actual motion of the vessel, while 
the screw has a pitch equal to the vessel’s motion in addition to its own slip. 
If, therefore, we could construct the part of the screw nearest the centre in 
the form of the helices thus generated, we might make the screw with threads 
continued from the axis to the periphery, but without diagonal loss near the 
centre; and to approximate to this as much as possible, the writer proposes to 
decrease the pitch from the periphery to the axis, so as to make the parts near the 
middle coincide with the vessel’s motion. Such a form the method of constructing 
the screw, described at page 45, affords the means of easy execution. We consider 
its chief value to consist in its enabling us to attach the screw to the axis the 
whole of its length, as in the case in the screw of the Archimedes, but without 
the same loss of power.