EXPLANATION OF THE PLATES. 345 



PLATE XVI. 



Fig. 1. and 2. represent the instrument called the indicator, for measuring the 

 force of the steam in the cylinder of an engine. See art. 560. 



Fig. 3. and 4. are diagrams to illustrate the comparative stability of opposite 

 classes of forms for vessels. See art. 599, 600. 



Fig. 5. If the motion of a vessel were always direct, its sides should be parallel, 

 and one of the section Fig. 3. may be terminated by making both the extremities 

 of the same figure, and formed by circular arcs ; then if the section be similar, so 

 that the stability may be equal throughout the length, (art. 599.) the water 

 lines will increase in curvature towards the keel (they are shown by dotted lines) ; 

 but the actual obliquity of the resisting surface, by which these resistances are 

 measured, decreases in descending. The objection I should make to this mode of 

 forming a vessel is, that it would not have a sufficient tendency to keep in its 

 course ; and I think a better form would be obtained by conceiving the midship 

 section to advance parallel to itself, and also towards the keel, in the same manner 

 as is shown in the next figure. 



Fig. 6. If the section Fig. 4. be the midship section, and the plan of the load- 

 water line be formed by arcs of circles, and the sections be all drawn by the same 

 mould as the midship section, as far as the breadth at the part allows, then the 

 form will be as Fig. 6. ; the water lines would be all of the same curvature, the 

 capacity would be easily measured, and the construction would be simple. 



But it is necessary to remark, that parallel sides are best only for direct motion. 

 In an oblique motion, such as that almost universally produced by wind, the 

 vessel should diminish towards the stern ; the oblique force of the wind then 

 presses its side against the fluid so as to produce an effect similar- to that of an 

 inclined plane, if the sails be properly set, and I think the diminution should 

 commence where the curvature of the fore-part ends. 



It is chiefly for direct motion that a steam vessel is intended, and where it is so, 

 parallel sides have the advantage ; but where sails are to be used with effect in 

 addition to steam power, the direct resistance must be a little increased, or the 

 capacity diminished, to get a clean run when the oblique force of the wind is 

 available. Hence, it appears that a vessel adapted for one mode of action is not 

 the best for another ; and instead of theory being imperfect, it is evident that it 

 only wants to be followed up by analysing the different cases which occur in 

 practice. It is difficult to conceive how much this subject has been neglected, or 

 how much remains to be done. 



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