286 



NA TURE 



{Jan. 29, 1 ! 



farther ; but if it be above the centre of gravity she will 

 tend to return towards the upright position. Sir Edward 

 Reed objects to calling these points metacentres when 

 the angles of inclination are large, because they really 

 have " nothing to do with limiting the height to which 

 the centre of gravity can be raised without disturbing the 

 upright position of the ship." We do not see that any 

 such property is implied by calling the metacentre for a 

 certain angle of inclination the metacentre belonging to 

 that inclination, which is what the French do. There is 

 no doubt that the French method is a natural and useful 

 one. It is also one which has been adopted by many in 

 this country, and is likely to become general. M. E. 

 Bertin, of Brest, says that the nomenclature adopted 

 throughout France for many years past has been such as 

 to leave no room for difficulties of interpretation. But 

 while Sir Edward Reed objects to the French practice in 

 this respect, as well as to Prof. Rankine's " shifting 

 metacentre," he does not propose a substitute for either. 



There are other matters connected with the nomen- 

 clature of this branch of science which might have been 

 profitably dealt with by the author. Take, for instance, the 

 common use of the term stability. The stability of a floating 

 body is determined by the forces which resist its angular 

 motion from a position of equilibrium, and by the angular 

 distance over which such forces operate. What is called the 

 curve of stability is a curve of which the abscissa? denote 

 angles of inclination and the ordinates are proportional 

 to the righting moments. This may quite properly be 

 called a curve of stability, as it gives a complete graphic 

 representation of the various elements of stability. But the 

 righting moment possessed by a ship at a given angle of in- 

 clination is frequently called the statical stability at that 

 angle : while, by a still stranger misuse of scientific lan- 

 guage, the work required to be done to incline her from the 

 upright position of equilibrium to the angle in question is 

 called the dynamical stability at that angle. Stability 

 exists only at positions of equilibrium ; and it is absurd 

 to speak of foot-tons of stability at a given angle of in- 

 clination from one of those positions, as is frequently 

 done. Such mistakes can only be due to the confusion 

 which exists in many minds between stability and righting 

 moment. Prof. Osborne Reynolds called attention to 

 the point at the meeting of the British Association in 

 1883. Whether any intelligible meaning is supposed to 

 be conveyed by the words " dynamical stability developed 

 during inclination," we do not know ; at any rate, we 

 cannot discover what it is. Sir E. J. Reed has done 

 good service by approaching the question of mistaken 

 and ambiguous terms in naval architecture. We are 

 only sorry that he has not dealt more thoroughly with it. 



Sir Edward gives numerous illustrations of curves of 

 metacentres, and curves of stability, for various types of 

 ships ; so that the effect upon them of variation in the 

 proportions and form, and also in the loading, of various 

 types of vessels, may be studied. These curves are given 

 for broadside armour-clads, low freeboard turret-ships, 

 and armoured cruisers of our own and other navies ; and 

 for passenger and cargo steamers. The latter include 

 examples which show the character of the stability that 

 many vessels of the " well-deck" type possess. 



The subject of longitudinal stability is fully dealt with : 



and the effect of admitting water into a watertight com- 

 partment is discussed. The method of determining the 

 height of the longitudinal metacentre is explained ; and 

 also the moment required to alter the trim of a given ship 

 by a fixed amount. The changes of trim produced 

 by putting weights into or taking weights out of a ship 

 are clearly described. The stability of a vessel fitted 

 with watertight compartments, and having water admitted 

 into one or more of them by collision, or otherwise, is in- 

 vestigated with great fulness of detail. The following dis- 

 tinct conditions are considered : (1) When a closed com- 

 partment is completely filled with water ; (2) when a 

 closed compartment is partially filled with water ; and 

 (3) when a compartment contains water in free com- 

 munication with the sea, and in which the water 

 maintains the same level as the sea for all inclinations. 

 In dealing with this subject Sir E. J. Reed substantially 

 follows the lines laid down by Mr. F. K. Barnes, of the 

 Admiralty, in papers read before the Institution of Naval 

 Architects in 1864 and 1867. Mr. Barnes has very ably 

 and lucidly explained the effect upon the metacentric 

 height which is produced by laying a central compart- 

 ment in a ship open to the sea and filling it with water ; 

 and also the effect produced by thus filling compartments 

 which are formed by longitudinal bulkheads. The results 

 are given, in both cases, for compartments of various 

 sizes and proportions. 



Sir Edward devotes a chapter to the consideration 

 of " dynamical stability," and gives the views that have 

 been put forward respecting it by the late Canon Moseley 

 and by MM. Moreau, Bertin, Risbec, and Duhil de Benazd 

 He also quotes an interesting and ingenious investiga- 

 tion by M. Guyou, of the French Navy, which includes 

 a somewhat novel treatment of the problem of dynami- 

 cal stability. We have already objected to the use of 

 the phrase " dynamical stability." The author explains 

 that what is called the dynamical stability at a given 

 angle of inclination is the work done by an inclining force 

 in heeling the ship from the upright position of equi- 

 librium through that angle. The "total work is the 

 dynamical stability." Dynamical stability is consequently 

 spoken of as being " developed during the inclination of 

 the ship from one angle to another." Resistance is over- 

 come, and work is done, in inclining a ship from one 

 angle to another against the action of righting forces ; 

 but we cannot understand why such work should be called 

 " dynamical stability." 



The work done in inclining a ship from one angle to 

 another is, of course, the resistance to such inclination 

 multiplied by the distance through which the resistance 

 is overcome. This resistance is constituted by the weight 

 of the ship acting vertically downwards through its centre 

 of gravity, and an equal and opposite force acting verti- 

 cally upwards through the centre of gravity of the dis- 

 placed water. Therefore the total work performed during 

 an inclination is the weight of the ship multiplied by the 

 vertical increase of distance between the centre of gravity 

 of the ship and the centre of gravity of the displaced 

 water. 



This treatise contains an instructive chapter upon M. 

 Amsler-Laffon's mechanical integrator. The ordinary 

 pivot-planimeter, which is a more common and very valu- 



