GENERAL DEDUCTIONS. 
3 7 
resistance follow the same law as that which governs the direct resistance alone; 
that is to say, it is as the square of the velocity. It will be found that the results 
agree as near to this proportion as could be expected where the impelling force 
was steam power. Hence we conclude that the surface friction is also as the 
square of the velocity. It will be seen from the Table that the loss of power 
at 120 revolutions would be less than 5-horse power; which will be increased 
about one-third. by the increased surface in a screw. But it must be recollected 
that the paddles have also to encounter surface friction as each of them suc 
cessively dips into and emerges from the water. This cannot be so much as 
appears to be lost by the screw, because the velocity of the paddle is much less; 
but there is the direct resistance arising from the edges of the floats, and the 
rims and arms of the wheel, which should be taken into account: and though 
we have no experiments to guide us as to the total amount thus lost, it is 
probable it would not be much less than the loss with the screw from the same 
causes. Indeed, we had made a tabular calculation by which this fact was 
established; but where the parts are so numerous, and their action and resistance 
not capable of being clearly traced, there was rather too much assumed to render 
the calculation of any value, and the reader must therefore be contented with 
an inference of the fact. 
GENERAL DEDUCTIONS. 
The general deductions from the foregoing considerations appear to be,— 
First. That when the paddle wheel is at an ordinary immersion, and the slip 
equal in both systems, the advantage is somewhat in favour of paddles; but that 
when a vessel with paddles is deeply laden, or rolling considerably in a sea-way, so 
as to immerse the wheel beyond a certain dip, or when the screw is so constructed 
as to have less slip than the paddle, the advantage is in favour of the screw. And 
further, as a beam wind necessarily depresses the lee wheel so as to cause it to be 
disadvantageously immersed, a steamer with paddles can only partially avail of such 
a wind; while with the screw there is no such limitation, and canvass can 
consequently be used to the same extent as in a sailing vessel. The advantage 
possessed by the screw in this respect was clearly proved in the trials between 
the Archimedes and Her Majesty’s steamer Widgeon, where the latter vessel was 
superior to the former in calm weather or a head wind, but inferior with the wind 
a-beam. 
Secondly. That the maximum efficiency of the screw depends upon its velocity, 
area, and pitch being so proportioned that the sum of the loss by angular slip, 
and by surface friction, shall be reduced to a minimum quantity; for it is evident