A Treatise On The Principles And Practice Of Harbour Engineering

214 HARBOUR ENGINEERING. describe the quadrant ABC. Join AC. From C draw CE at right angles to HK, cutting the axis in the point E, which is the focus. Through D draw a horizontal line of indefinite length. Take any number of parallel lines intersecting the axis at right angles. Measure along the horizontal line through D the distance from D to each point of intersection. With these distances as radii respectively, and the focus as centre, mark corresponding intersections on the parallel lines, on both sides of the axis. These intersections are points on the curve. Note.—In the investigation on p. 210, the value of a was found to be — ^/LD To obtain the value graphically, take a line (fig. 188) whose length is L+D, and divide it into two parts equal to L and I) respectively. Üpon the line describe the arc of a semi-circle, and from the point of section draw a vertical linejo meet the semi-circle. This line has a length ^LD, and two- Fig. 188. thirds of it will give the value a. (III.) Gase of the ballasted pontoon.—We have so far only considered the pontoon as an empty box— independent and self-contained. We have now to regard it in its working aspect. It is intended to carry a load, and for purposes of insertion and withdrawal from position it is oftentimes ballasted with water. The object of admitting water to the interior of the pontoon is to enable it to be lowered temporarily to a deeper draught, from which it can be raised again by pumping out the water. We will deal first with the matter of ballasting. When the pontoon is a single box, without compartments, the introduction Fios. 189, 190. of water diminishes its stability, as will be evident from the diagram. The fluid, instead of remaining equally distributed under disturbance, immediately flows to the deeper side and there assists the overturning moment by its impetus, or, at least, impedes the righting effort. The buoyancy, moreover, of the pontoon is reduced by the occupation of internal space. The drawback of shifting ballast may be to some extent mitigated by subdividing the pontoon into compartments. When disturbed, the distri- bution of the water becomes less markedly unequal, as is evident from figs. 189 and 190. The greater the number of compartments the more uniform