SHIP BUILDING. 37 



Another kind of rolling, namely that in the direction of the axis of the 

 ship, is pitching, in which the bow of the ship rises and falls. A wave 

 meeting the ship raises the bow, which falls again as soon as the wave has 

 passed, and this action is repeated with every new wave. If a ship is close 

 to the wind, it often happens that when a wave has passed the forward part, 

 the bows fall rapidly and only rise with difficulty at the approach of the 

 next wave ; in this case the ship is said to pitch {pi. 21, jig. 2). When a 

 wave has passed the forward part of the ship and arrived towards the 

 centre, a considerable portion of the ship's body is without support. This 

 portion falls upon the surface of the water with a considerable degree of 

 force, composed of the product of the weight of the whole forward part and 

 the length of the unsupported part. Sometimes this motion takes place at 

 the stern, and then the ship is said to fall. Both cases are equally unplea- 

 sant in their effects, as they diminish the rapidity of sailing and expose the 

 ship to great danger. The defects which the ship-builder has fallen into 

 in this respect may often be partially remedied by experienced seamen, if 

 they take great care in the stowage and place the greatest weight in the 

 centre of the ship. 



7. Fastening the Body of the Ship. We know from common experi- 

 ence how difficult it is, even in the most simple carpentry, to preserve the 

 shape of a building, and we are hence obliged to use a variety of braces and 

 supports. But in ship-building the preservation of the form is far more 

 important and more difficult, as the greatest danger would arise if the 

 ship on leaving the stocks should become loose in the different parts and 

 not retain its prescribed form. We have examples of such accidents. In 

 ship-building especially theory and practice must go together. It is not 

 only the violence of storms which tends to disturb the form of the vessel, 

 but the pressure of the water even when quiet, which properly sustains the 

 ship, exercises a similaf force. If we draw a straight line from the stern to 

 the stem of the ship, while she is still on the stocks, this line will often be 

 deflected some five or six inches as soon as the ship touches the water. 

 This is owing to want of precision in the work. Whole planks and con- 

 necting pieces are often forced out of place and broken. The length of a 

 seventy-four is 170 feet or more, and only a slight knowledge of the strength 

 of materials will show that in so great a length the strongest timber must 

 bend under its own weight, and a change of form, therefore, is almost unavoid- 

 able. Seppings, one of the best English ship- builders, has endeavored to 

 avoid this difficulty by the plan of oblique bands. We know that a mere 

 quadrangle can never be firmly put together, but that the simplest lattice- 

 work must have an oblique band, in order to hold its shape. If we compare 

 pi. 7, jig. 5, which represents the old system of ship-building, with jig. 6, 

 which indicates the main principles of the new, we shall perceive that the 

 advantages of Seppings's plan are in proportion to the lengths which we have 

 to deal with. The effect of the triangular system is to give the pressure in 

 the direction of the fibres of the timber, while in the rectangular system the 

 strain comes across the grain. PL 0,fig. 1, shows an interior view of the 

 side of a seventy-four according to Seppings's system, where t'jj diagonal 



iconographic encyclopaedia. — VOL. III. 44 689 



