642 Professor Andrew Gray [Feb. U, 



Anotlier mode of motion is possible which has a very intimate 

 connexion with the theory of vibrations of light-emitting molecules 

 in a magnetic field, as indeed I pointed out here several years ago in 

 a Friday Evening Discourse (see "Nature," April 13, 1899, and August 

 24, 1899). The bob can be made to move in a circle about the 

 vertical through the point of support either with or against the direc- 

 tion of rotation of the flywheel. The two periods are different, and 

 the motions correspond to the circularly polarized light of two dis- 

 tinct periods, which molecules, situated in a magnetic field, are found 

 to emit. Thus the gyrostatic pendulum gives a dynamical analogue 

 of the cause of the Zeeman effect. 



In 1907 Herr Otto Schlick introduced a method of employing 

 a gyrostat to counteract the rolling of a vessel at sea. The gyrostat 

 is carried on bearings placed athwart the ship. These bearings are 

 in line with the flywheel, and a weight is attached to the frame of 

 the gyrostat in a position in line with the axis. It will be seen that 

 when the ship is on even keel the gyrostat rests with its axis vertical, 

 and with the weight vertically below the centre of gravity of the 

 flywheel. Heeling of the ship in one direction causes the gyrostat 

 to precess in one direction on the bearings on which it is mounted ; 

 heeling in the other direction causes precession in the opposite 

 direction, and couples resisting the rolling motion are brought to 

 bear on the ship. The device may be employed in two ways. In 

 the first place, if the bearings on which the frame of the gyrostat is 

 carried within the ship are smooth, the effect of the gyrostat is to 

 resist the rolling force of the waves, and to bring about a lengthen- 

 ing of the free period of the ship, according to a mathematical theory 

 which, when put in the proper way, is really very simple. Excessive 

 rolling of a ship is due to the cumulative action of the waves, and 

 such cumulative action is only possible where the period of the ship 

 and that of the waves are of about the same order. A large ship has 

 a very long period, and synchronism of the ship and the waves is 

 impossible. The effect of introducing a gyrostatic control, operated 

 in the manner just described, is to endow the small ship with the 

 period of a very large one. 



In the second mode of operating the gyrostat, friction is intro- 

 duced at the bearings on which the frame of the gyrostat is mounted. 

 With this addition the ship is forcibly prevented from excessive 

 rolling. In the trials of the device it was found that, with the 

 control in operation, the angle of roll of the ship did not exceed 1° 

 in a cross-sea which produced a total swing of 35° when the control 

 was out of action. It is interesting to notice that, contrary to the 

 opinions which were expressed when the device was first suggested, the 

 preventing of the rolling of a ship does not result in the waves break- 

 ing over her ; a ship controlled by a gyrostat is, I believe, a dry one. 



I have here a motor-gyrostat fitted within a skeleton frame 

 representing a ship (Fig. 12). The frame is mounted on two bear- 



