It is the judicious and proper union of theory and practice that is wanted to carry this very important art onwards to perfection. "Practical knowledge alone would be insufficient, nor would the highest the oretical skill be all that would be required. The two must be united,—cordially and harmoniously united. Practice must not decline the assistance of theory, nor must theory disdain to be taught by the lessons of practice."1- "There are many principles," says Air. Atwood, "deducible from the laws of mechanics, which it is probable no species of ed.periment, or series of observations, however long continued, would disco ver; and there are others no less important, which have been practically determined with sufficient ex actness, the investigation of which it is scarcely pos sible to infer from The laws of ?notion; the complicated and ill-defined nature of the conditions, in particular instances, rendering analytical operations founded on them liable to uncertainty." It is true indeed, as the same writer remarks in another place, that "although all results deduced by strict geometrical inference from the laws of motion, arc found, by actual expe rience, to be perfectly consistent with matter of fact when subjected to the most derisive trials, yet in the application of these laws to the subject in question, difficulties often occur, either from the obscure na ture of the conditions, or the intricate analytical ope rations arising from them, which either renders it impracticable to obtain a solution, or, if a result is obtained, it is expressed in terms so involved and complicated, as to become in a manner useless as to any practical purpose. These imperfections in the theory of vessels are amongst the causes which have contributed to retard the progress of naval architec ture, by increasing the hazard of failure in attempt ing to supply its defects by experiment; for when no satisfactory estimate can be formed from theory, of the effects likely to ensue from adopting any altera tion of construction that may be proposed, doubts must unnecessarily arise respecting its success or failure, which can be resolved only by having recourse to ac tual trial; a species of experiment rarely undertaken under the impressions of uncertain success, when the objects are so costly, and otherwise of so much im portance. To the imperfections of theory may also be attributed that steady adherence to practical methods, rendered familiar by usage, which creates a disposition to reject, rather than to encourage pro posals of innovation in the construction of vessels: the defects or inconveniencies which are known, and have become easily tolerable by use, or may perhaps be the less distinctly perceived for want of comparison with more perfect works of art, being deemed prefe rable to the adoption of projected improvements, attend ed by the danger of introducing evils, the nature and extent of which cannot be fully known. These are amongst the difficulties and disadvantages which have concurred in rendering the progress of improvement in the art of constructing vessels extremely slow, and left many imperfections in this practical branch of science which still remain to be remedied."* Another important consideration which has tended to retard the progress of naval architecture, is the immense variety of vessels, which the peculiar cir cumstances of climate, varieties in the extent and depth of waters and of seas, have rendered necessary to man; each variety differing in proportion and form, in their methods of rigging, and in their modes of navigation; some being adapted for limited voyages in narrow and contracted channels, others for voyages the most extended in the widest oceans; some for winds of almost a permanent character, and others for all those uncertain varieties of weather, which mark so many of the regions of the earth.
Amidst this almost infinite diversity, we may how ever trace, in numerous instances, indications of something like general laws. In those vessels, for ex
ample, which are destined for extended voyages, we find their extreme breadths to he between a third and fourth of their lengths. In vessels of a smaller size, the breadth bears a greater proportion to the length, than in ships of a higher class. The elevation of the deck above the surface of the water has likewise li mits, which are regulated by the peculiar destination of the vessel. All ships, moreover, have their maxi mum of breadth, a little before the middle; the forms of their forward and after parts are variable, but still distinguished in all cases by this peculiar feature, that the figure of the latter part is more slender, or as it is technically expressed, is leaner than the former. In ships destined to hear heavy burdens, the bottom is fuller to admit of greater capacity for stowage, in opposition to those which are built for speed and ve locity, which have uniformly their lower parts of a sharper figure. The line of the keel also does not run in a plane parallel to the fluid surface, but has a great er draught of water in the after-part of the vessel, than in the forward; that both the stern and the stern have a rake or inclination between certain limits; that in the rigging of ships, some have one mast, others two, and most three masts, adapted in their diameters and altitudes, to their peculiar circumstances; that the centres of gravity of all vessels are found a little before the middle of their lengths, and that the cen tre of gravity of the sails is uniformly met with before the centre of gravity of the ship.
Thus it appears, that amidst the seeming diversity characterising the numerous branches of this very important subject, there are indications of genera] laws to be found, which experience, in a long suc cession of ages, has taught the navigator to follow, in order to succeed in his adventurous enterprises. And it is remarkable in how many instances the results of uneducated men have anticipated the soundest deduc tions of the most enlarged theories; and how uncon sciously they have employed, even in their pastimes and sports, those very principles on which the philo. sopher raises the noblest monuments of his fame. " The savage who never heard of the accelerating force of gravity, yet knows," says Mr. Stewart, " how to add to the momentum of his missile weapons, by gaining an eminence; though a stranger to Newton's third law of motion, he applies it to its practical use, when he sets his canoe afloat, by pushing with a pole against the shore; in the use of his sling, he illus trates with equal success, the doctrine of centrifugal forces, as he exemplifies (without any knowledge of the experiments of Robins) the principle of the rifle barrel, in feathering his arrow." And just so is it in the steps which have marked the progressive advance ment of naval architecture. The practical knowledge which the framer of the canoe exercises, " is obtruded on the organs of external sense by the hand of nature herself." He found, for example, that a particular disposition of the sail of his little bark would give to it a greater velocity than any other. A change of position of his own body, or of a stone in the bottom of the canoe, would alike influence its sailing quali ties. These to him would he maxims of great prac tical value; would be treasured up and recollected, ap plied on every necessary occasion; in time communi cated to his fellow-navigators, and at last identified with the general habits of his tribe. From such be ginnings it is, that naval architecture has arisen; and the philosophical observer endeavours to draw from the maxims which have guided even uneducated men in a long course of ages, those general laws, which, when moulded into a systematic form, enable him to perceive relations still more extended and general.