214 REPORT — 1873. 



stably upright in still water, slie is for the moment iu equilibrium if uprig-ht or 

 normal to the mean or eflective slope of the wave which she occupies ; and if she 

 have a given righting force when inclined to a given angle in still water, she will 

 be urged by approximately the same righting force towards the normal position in 

 wave water if she at any moment deviate from it by the same inclination. (2) A. 

 plumb-line or pendulum, if its point of suspension be at or very near the ship's 

 centre of gravity, will hang at rest if it occupy the normal position, and if it have 

 a very short period of oscillation it will instantly assume that position throughout 

 the changes of the wave-slope. The apparatus in question might be tlius described. 

 A revolving cylinder covered with paper and turned by rough clockwork received 

 the marks made by several pens. One of these pens recorded time, jerks being 

 given it at successive equal intervals by an exact clock. The apparatus being 

 placed at the centre of gravity of the ship, a pendulum of very short period and 

 considerable power, oscillating in the plane transversely with the keel, recorded 

 continuously by a second pen the angles which the ship at eacl? moment made 

 with the mean or effective surface of the wave. Another pen actuated by a 

 rocking-arm kept level by an observer on deck, wlio pointed it to the horizon, 

 recorded the angle the ship made with the horizon ; and from the record thus 

 obtained the amount of the roll of the ship with regard to the wave-slope was at 

 once shown : the form of the wave, too, could be easih' worked out graphically, the 

 wave-slope at each moment being simply the difterence between the records pro- 

 duced by the pendulum-pen and the horizon-pen respectively. But the gi-aphic 

 integration of the results supplied by the pendulum-pen, if correctlj' performed, 

 supplied what might be called the theoretical measure of the oscillations which 

 the ship ought to have performed with regard to the horizon during the period 

 embraced in the record ; for the pendulum record itself supplies throughout a 

 measure of the accelerating force by which the ship's oscillation is governed ; so 

 that the integration of this gives a diagram representing the angular velocity which 

 the ship should theoretically have acquired under the operation of that force ; 

 and the integration of the velocity diagram in turn gives the sequence or total of 

 motions which the varying velocitj' involves. The performance of these integra- 

 tions involves, indeed, a con-ect knowledge of the ship's dynamic constants ; but 

 these, so far as they are not already known by calculation, may be readily 

 obtained by a single experiment with the ship in still water, where, if she be 

 artificially brought into oscillation (an operation easily performed), and the instru- 

 ment be made to record the oscillations as they subside under the iniluence of 

 resistance, the natural period of her oscillation is at once known, and the coefficient 

 of resistance is deducible in a shape which is approximately applicable to the ship's 

 seaway oscillation. All the conditions required for the integration are thus 

 supplied. Several series of diagrams thus obtained by the oscillation of ships 

 in a seaway had been thus integrated, and the theoretical oscillations accorded 

 with the recorded oscillations, so that the fundamental elements of the theory of 

 rolling had been most satisfactorily veiified. An apparatus had also been com- 

 pleted consisting of a heavj' stationary wheel, which was so delicately supported as 

 to be incapable of receiving any rotation from the motion of a ship. ' This wheel, if 

 placed transversely in the ship, would remain quite undisturbed while she rolled, 

 and would thus supply the place of the horizontal bar above described, held 

 level by the observer on deck. The wheel was 3 ft. in diameter and 200 lb. in 

 weight. Through the boss was carried out a strong steel axis, the prolonged ends 

 of which were coated with hardened steel. The axis thus prolonged rested 

 between two pairs of rocking-arms, the ends of each pair fomiing a kind of V- The 

 ends of the aims were, in fact, hardened steel plates, fonning segments of circles 

 struck from the axes or centres on which the arms rocked, so that they were 

 virtually portions of the circumferences of very large friction-rollers. In order still 

 further to reduce the friction of the working parts, the axes of the rocking-arnis 

 were finally reduced to hardened steel pins of small diameter, and so mounted that 

 their motions when of small range should be rolling not sliding motions, and great 

 delicacy was thus obtained. The centre of gTavity was brought to within OC0(].5 in. 

 of the axis of suspension, and the time of a single swing was over thirty-five 

 seconds : yet £o great was the delicacy of the guspension, that a weight of aioVoir part 



