46 
MODE OF COMMUNICATING MOTION 
It has also been proposed to make the screw of wood and iron combined, and the 
writer proposed a method of thus constructing it. This was by placing iron arms 
upon an axis, as in the Archimedes’ screw, filling up the spaces between them 
with pieces of wood, and binding the periphery round with a sort of hoop. The 
whole surface of the wood segments was then intended to be dressed off smooth to a 
template. 15 This would form an exceedingly light and inexpensive screw, but 
it would not possess the accuracy we have shown to be so desirable, and on the 
whole we should prefer casting the screws to constructing them in any other way. 
ON THE MODE OF COMMUNICATING MOTION FROM THE ENGINE TO THE SCREW. 
We have hitherto been investigating the screw without reference to the method 
by which it is to be driven. This, however, is by no means a secondary con 
sideration, as whatever may be found to be the ultimate efficiency of the screw 
compared with the paddle, there is this wide distinction. In paddle wheels the 
velocity of the engine, the periphery of the wheel, and the speed of the vessel, 
may be so nicely proportioned that motion may be, and invariably is, com 
municated from the engine to the paddle shaft without the intervention of any 
machinery for changing the speed, whilst the screw requires such a high degree of 
velocity as to render it necessary in every case to adopt some mode of multiplying 
the motion of the engine shaft. Hence it becomes essential to consider the effect 
which such multipliers have in adding to the complication of the machinery and 
on the durability of the whole. The known methods of giving such increased 
velocity are 
Toothed wheels, 
Bands or ropes, 
Contact, by pressure, of smooth surfaces. 
On the action of toothed wheels there is nothing new to be said. There are 
instances of these having been in operation for thirty years without repair, of 
which those furnished to the machinery of the Royal Mint by the late Mr. 
Rennie are an admirable instance, the same cogs being still in good order and 
efficient. 
But, although we might multiply instances of the durability of toothed or 
cog wheels, it must be admitted that they can never be so secure as the direct 
action of the paddle shaft. The risk of injury to the engine shaft is common 
to both the paddle wheel and the screw, but is increased in the case of the 
15 Since the above was written, the author understands that this is the method adopted in a vessel 
built by Mr. Whimshurt, which is propelled by a pair of non-condensing engines.