82 
WATER AND WATER NAVIGATION. 
be driven beyond a certain fpeed, it cuts out the water 
contained within its blades: the blades and the water 
revolving as a folid mafs. Under these circumftances the 
propelling power of the fcrew is diminifhed rather than 
increafed. * * * The vanes or blades of the fcrew, 
as commonly conftruded, are fixed at a given angle, and 
confequently always ftrike at the fame degree of ob¬ 
liquity. * * * In this arrangement power is loft, the 
two vanes ftriking after each other in the fame manner, 
in the fame diredion, and almoft precifely at the fame 
moment—no provifion being made for increafing the angle 
and the propelling power at one ftage of the ftroke, and 
reducing it at another to diminifti the amount of flip, 
incidental to the arrangement.” 
I think no one can doubt the corrednefs of the above 
criticifm: and the defeds appear to me beyond remedy fo 
long as the propeller is a fcrew. 
Fifties propel themfelves by the tail and flexible por¬ 
tion of the trunk : and fcrew-propulfion is defigned to imi¬ 
tate this method—and may be called the fish method. 
But aquatic birds that purfue their prey under water— 
fome even going to the bottom of rivers—fwim with their 
wings . Thefe creatures are a fort of “ fide-wheelers.” 
And both plans work well.—Why, then, confine ourfelves 
to the fiern-power method P 
If a veflel were conftruded with receptacles or con¬ 
cavities in the fides made within the water-lines and fur- 
