2i8 ACTIVE TYPE OF STABILIZING GYRO. 



dampen in the ordinary sea conditions. Even if we assume that there will be heavy 

 rolling of the ship without resonance between the periods of the waves and the 

 period of the ship, this, of course, would only be true if the ship itself should not 

 retain its individual rolling period, to which the period of the water in the tanks 

 has been adjusted. As soon as it is proved that the ship always has the same or 

 nearly the same period, the effectiveness of the tanks would not be affected by 

 the absence of the primary resonance between waves and ship. Now on page 201 

 Mr. Sperry doubts the fact of a constant rolling period of the ship, saying that he 

 noted a change of 30 per cent between the period of a ship at 15 knots and at 

 o knots, but he does not say anything on the law governing the dependence of the 

 rolling period upon the speed. Similar doubts were uttered during the discussion 

 before the English Institution of Naval Architects in 191 1, which dealt very largely 

 with these questions, and from which Mr. Sperry quotes at different times in his 

 paper. At the end of the discussion, Herr Erahm said: — "I have had before me 

 quite a number of rolling diagrams recorded on board of all sorts of ships, tender 

 and stiff, which proved the correctness of my assertion (as to ships rolling only in 

 their individual period). In fact all these ships, the displacement of which varies 

 from 1,200 to 30,000 tons, when rolling considerably have always maintained their 

 individual period, and only trifling differences, which rarely came to more than 

 five per cent, have been observed." There may be considerable differences, 

 however, in the period of the waves, in resonance with the period of the ship. 

 This may easily be seen when considering that the ship, steaming against the 

 waves at a certain angle, receives more impulses from waves of a certain period 

 than a ship struck by oblique waves of the same period coming up from behind. 

 In both cases, however, the period of the ship remains the same, and the tanks 

 therefore will be effective. 



If Mr. Sperry denies the existence of primary resonance, there is still another 

 question to be considered: — Is the ship rolling at all to a considerable amount if 

 there is no resonance between the waves and ship? Since the ship as a whole is 

 rolling in exactly the same way as an ordinary pendulum, the simple, well-known 

 experiment of the double pendulum will prove to you that there will be no heavy 

 rolling if there is no primary resonance between waves and ship. In this experi- 

 ment the second pendulum is only swinging heavily if it has the same period as 

 the first pendulum from which it receives the impulses. The second pendulum 

 will make small, irregular oscillations only, in both cases, if its period is shorter 

 or longer than that of the first pendulum. During the reading of his papers before 

 the German Schift'bautechnische Gesellschaft, 19 10; the EngHsh Institution of 

 Naval Architects, 1911; Herr Erahm showed an experiment in attaching a third 

 pendulum to the second, the first pendulum, which received regular impulses, now 

 representing waves of a given period, the second representing the ship and the 

 third the water in the tanks. By changing the length of the second and of the 

 third pendulum and thus establishing conditions of resonance of shorter or of longer 

 periods of the ship and of the tank water, Herr Erahm clearly showed that all 



