12 6 Mr. Atwood’s Propositions determining the Positions 
which the forms of vessels and the disposition of their most 
essential parts are ascertained, theory may be considered as 
bearing to naval architecture a two-fold relation : first, as de- 
pending on the pure laws of mechanics, a subject on which 
the preceding cursory observations have been offered : se- 
condly, the practice of naval architecture is guided, in most 
parts of the world, by a species of theory or systematic rule 
which individuals form to themselves from experience and ob- 
servation alone : it is founded on the experimental knowledge 
in naval constructions, which has been transmitted from 
preceding times, combined with the more recent improve- 
ments, and includes whatever inventions of skill and in- 
genuity are applicable to the various machinery that is em- 
ployed in the construction and management of vessels : by re- 
peated observation on the forms, proportions, and equipment 
of ships, and by attention to their excellencies and defects 
when afloat at sea, faults are remedied, good qualities are im- 
proved, and rules of practice are by degrees established ac- 
cording to principles, well perceived and understood, without 
much assistance from the theories of mechanics, statics, and 
geometry, on which such principles are founded : for in this, 
as well as other instances, it is well known that skilful practice, 
aided by long experience, arrives at determinations which it is 
very difficult (sometimes impossible) for theory to infer: on 
the other hand it must be allowed, that pure theory, depend- 
ing on the laws of motion, the subject of disquisition in 
gation, have been subjected to practical examination during voyages. M. Chapman, 
in page 79 of his work (Paris edit.), expresses the proportions and disposition of parts 
in vessels by algebraic quantities, which, however, are not to be mistaken for deduc- 
tions from theory j since the author has not pointed out any mode of investigation, or 
train of reasoning, by which those expressions can be deduced from the principles of 
mechanics! 
