1823.] On the Construction of Sea Harbours^ I^ 



Article II. 



Essaifs on the Construction of Sea Harbours. 

 By Mr. J. B. Longmire. 



iConiimicd from vol, v. p. 182.) 



(To the Editor of the Annals of Philosophi/,) 



SIR, Troutheck, May 20, 182S. 



On the following phenomena of fluids, in direct and reflected 

 motions, depencj the disposition of the piers ; which is perfect, 

 when the water in the harbour is still, or nearly still ; the surf, 

 as httle increased as possible by the piers ; and when they inter- 

 fere not with the lines of approach. 



a. Straight waves, driven by the wind, directly through an 

 opening into the still water of an harbour, assume curved 

 figures ; which, as they advance, become nearly semicircular, 

 increase in length, and decrease in depth, till they are quiescent. 

 Waves obhquely driven through a given opening do not agitate 

 the interior water so much as direct waves ; for the length of 

 the waves that can pass through decreases as the angles of obli- 

 quity increases. 



The waves, in moving over the surface, give the water under 

 them impulses that create an agitation, extending much beyond 

 them. This, which is here designated lateral agitation, to dis- 

 tinguish it from the surf, acts in and near harbours through the 

 whole depth of the water, and appears on the surface in flat and 

 slow undulations. 



If a part of the harbour be separated by an inner pier, which 

 only leaves a small entrance, not far from the principal one; 

 then small parts of the waves pass into this bason ; but being 

 previously much decreased in height and velocity, and the 

 lateral agitation considerably weakened, they very little disturb 

 the water in it. 



The surf, driven by a strong gale, through an entrance facing 

 the sea, and sixty yards wide, provided the pier heads be oppo- 

 site and parallel, requires a space equal to 1 ,200 square yards, to 

 be quieted so much, that the rest of the bason have not undula- 

 tions larger than one foot high. 



When a gale commences, the waves but slightly disturb the 

 interior water ; yet by reiteration, the agitation greatly increases. 

 Now the art of stilling the water in a harbour that admits the 

 surf is to allow space for it to dissipate, and so to proportion an 

 entrance to the area of an inner bason, that when the agitation 

 is at its utmost, the undulations at the surface, where vessels lie, 

 ghall never exceed 15 inches, nor be repeated oftener than fivQ 



