RECESSION OF THE SCREW. 
33 
It appears from these calculations, therefore, that when the paddle wheel is 
most favourably immersed, if we exclude other deductions to which both the 
paddle and the screw are subject, the difference between the power and effect 
on each stands thus: 
Power. Effect. 
Paddle wheel 13*960 : 10*820 9 
Screw 71*576 : 48*875 
These Tables show that a paddle wheel at the ordinary immersion and slip 
would be (apart from other considerations) superior to the screw when the latter 
has the same slip; but if we sufficiently decrease the proportion of the slip of the 
screw the effect will be superior; and when the vessel is heavily laden or rolls 
in a sea-way so as to immerse the wheel up to the line m n, fig. 24, the screw would 
have an advantage over the paddle wheel. 
SLIP OR RECESSION OF THE SCREW. 
It may be imagined that in instituting a comparison we ought to assume a 
similar decrease in the slip of the paddle wheel to that proposed for the screw. 
Such decrease, however, cannot be admitted in practice: in the first place, because 
the outer edge of the paddle must necessarily recede considerably, or otherwise 
the inner edge would advance with the vessel when immersed, and retard instead 
of propelling her; and also because the floats cannot be increased in their esta 
blished proportions of breadth without increasing the loss by angular resistance. 
On the other hand, the slip of the screw is almost entirely a question of 
magnitude; for if we increase the diameter without altering the pitch, we reduce 
the slip without increasing the velocity at which it would be required to drive 
the screw. We should, however, add to the surface friction, on the amount of 
which we shall hereafter have occasion to remark. 
In the advanced state of our experience, derived from the working of paddle 
wheels of almost every conceivable form and proportion, we are enabled to approxi 
mate very nearly to the amount of loss by slip or recession of the water. But we 
cannot, without further experience, arrive at the same degree of knowledge with 
the screw; for, independent of our experiments being limited to few instances, 
a curious fact has been ascertained which seems to preclude us from arriving at 
any correct practical estimate thereof. A series of trials of the Archimedes 
9 The performance of the paddle called “ Morgan’s wheel,” as well as that of the cycloidal or divided 
paddles, both of which are the invention of the writer, will give a higher proportion of effect to the power 
consumed, but not so great as to render them equal under all circumstances to the screw. 
F