44 
ON THE MATERIAL OF THE SCREW. 
due weight; we therefore conclude our remarks on this part of our subject by the 
following quotation: 
“ The Archimedes has but one propelling screw, which is placed longitudinally in a hole 
cut in the dead wood immediately before the rudder, the keel being continued along under 
the screw. I conceive this to be the best position in which the propeller can be placed, 
for many reasons. It is more out of the way, less liable to injury, and less exposed to broken 
water than it would be in the bow—its action upon the rudder materially augments the 
power of the helm—and, upon reversing the motion of the propeller, the vessel runs stern 
foremost against a strong wind and rapid tide as freely as if she was going a-head. Moreover, 
a hole in the dead wood deprives a ship of little or no stowage; while placing the screw 
forward, amidships, or under the counters, would necessarily occupy greater space. 
“ It is probable, that as the water is driven with great violence from every part of the 
circumference, a considerable quantity of the fluid is lifted by the rotary action of the screw, 
so that the propeller may be said in some degree to supply itself, or at least to cover itself 
from observation. For this reason, I believe, in very shallow iron boats the screw would 
never be visible, even if it should not be wholly submerged at starting, though its propelling 
power might be slightly diminished. 
“ Having crossed the Atlantic eight times in all kinds of weather, I am perfectly aware of 
what description of seas a ship might have to encounter upon the ocean; but nevertheless 
I am persuaded, that in large vessels with deep draught of water, no pitching, rolling, or 
lurching of the ship, would be sufficient to uncover the screw, or to affect materially the 
regularity of its revolutions.” 
ON THE MATERIAL OF THE SCREW. 
We have described at page 15 the mode of constructing the screw of the 
Archimedes. This method, however, is expensive, and cannot be mathematically 
accurate. The surface friction, therefore, would be considerably increased by 
the thread in some degree departing from the true figure. It is also liable to 
corrosion in the inside between the plates and at their joinings. An iron screw is 
likewise objectionable in a coppered vessel, from the destructive effect produced by 
the galvanic action of the copper on the iron, the latter protecting the former 
by being itself decomposed. It will, therefore, be necessary to use iron screws 
in iron vessels, and gun-metal screws in coppered vessels. Under such circum 
stances the writer proposes in all cases to cast the screws. This can be done in 
such a manner as to produce the perfect figure from the mould. Such a screw 
was made under the writer’s superintendence by Messrs. Rennie, and as it was 
much admired for its accuracy and the method by which it was constructed, the 
process cannot fail to be interesting. 
The pitch and diameter of the screw having been determined on, two plates