Mr T. Tredgold on Steam-Boats . &51 
quit it ; the paddles are also more distant from one another, and 
while they have more re-action on the water, they splash it about 
much less ; the weight of the wheel also renders it more effec- 
tive as a regulator of the forces acting upon it. On the con- 
trary, there are some strong practicable objections to very large 
wheels for sea-vessels ; they give the force of the waves a great- 
er hold on the machinery, they are cumbersome and unsightly, 
and they raise the point of action too high above the water-line ; 
so that the choice requires both experience and judgment. 
The best position for the paddles appears to be in a plane 
passing through the axis, as represented in the figures ; if they 
be in a plane which does not coincide with the axis, they must 
either strike more obliquely on the fluid in entering, or lift up a 
considerable quantity in quitting it. With respect to the shape 
of the paddle, it is clear that it should be such that the resist- 
ance to its motion should be the greatest possible ; and the pres- 
sure behind it the least possible. These conditions appear to be 
fulfilled in a high degree by the simplest of all forms, the plane 
rectangle; but we might learn much from a judicious set of ex- 
periments on this subject. 
As there is some variation in the force of re-action against the 
paddles, it may in some measure be compensated by making its 
periods coincide with the variation in the force of the engine. 
To effect this, the stroke of the engine should be made in the 
same time as is occupied by that part of the revolution of the 
paddle-wheel, which is expressed by a fraction having the num- 
ber of paddles for its denominator, and the piston should be at 
the termination of its stroke^ when one of the paddles is in a 
vertical position. For, when one of the paddles is in a vertical 
position, as in the wheel A, Fig. 2., the re-action is the least, 
and it is greatest when two paddles are equally immersed, as in 
the wheel B, at which time the force would be acting at right 
angles to the crank. 
Having shewn the power that is necessary to keep a boat in 
motion in still water, it will be some advantage to resume the 
inquiry in the case where it moves in a stream or current ; and, 
for that purpose, let v be the velocity of the boat, and c the ve- 
locity of the current ; a being the resistance, when the boat is 
in motion with the velocity u. 
