INTRODUCTION. 
To investigate the properties of a new discovery in mechanics, or one of which 
the application is in its infancy, is, it will be conceded, a task of no ordinary 
difficulty. The deductions derived from actual practice (especially where our 
experience is not confined to a small number of illustrations) can be offered 
without fear of error, and almost the only onerous part of an author’s duty is 
to be certain that his facts are accurately stated. But where, as in the present 
instance, we are obliged in a great measure to substitute hypothetical inferences 
for practical data, the profoundest skill does not insure infallibility. 
It is therefore with more than ordinary hesitation that the writer of this article 
has undertaken the task for which Mr. Weale has done him the honour to 
select him, under the impression that his attempts to improve paddle wheels 
may have in some degree qualified him for the duty of investigating a subject 
so intimately connected with the same department of mechanical science. 
In submitting the following pages to the profession, and to those of the public 
who are interested in an inquiry of such vast interest, the writer cannot hope 
that his analysis of the properties of screw propellers, or his estimate of their value 
as compared to paddle wheels, may not be found, when our experience is more 
matured, in some degree imperfect and unsatisfactory. There are so many new 
phenomena involved in the action of a machine whose operation has been heretofore 
comparatively unknown, that its properties and powers cannot be fully inves 
tigated until more extensive and varied experiments (the only safe means of 
arriving at right conclusions) have made us acquainted with them. Even our 
knowledge of the paddle wheel, notwithstanding the length of time which it has