24 Mr. Sidney G. Brown [Jan. 80, 



Some half hour lief ore the lecture started, the compass was de- 

 flected from the north position, and it has since been left to itself. 

 Y<<u will see by the record that it is engaged in swinging back again 

 fco the north, recording a curve upon the paper strip. 



You will be able to follow the record in this way during the 

 whole of this lecture, as you cannot really look down upon the 

 compass itself. 



The compass is working two repeaters, these repeaters truly 

 copying the reading of the master compass. 



of course, any number of repeaters could be used on board ship 

 if it were necessary. 



Fig. '1 is the steering repeater ; it has a card that revolves four 

 times to one of the master, and the divisions are therefore very much 

 enlarged. 



The other repeater is a correction repeater, which I shall tell 

 you about later ; it has no magnified scale ; it is moving backwards 

 and forwards very slightly, and this motion we term the "hunt." 

 In the steering repeater the "hunt" has been cut out by providing 

 the mechanism within the case with a requisite amount of slack- 

 ness. 



About sixty-eight years ago Foucoult did what was thought a 

 wonderful thing at the time : he gave a lecture-room proof that the 

 earth was rotating on its axis — he looked through a microscope at a 

 gyrostat. He could not get a frictionless free vertical axis, so that 

 the experiment could not last for long. 



Presently I shall show you a piece of apparatus which carries out 

 Foucoulfs idea in a very perfect way, and it will be visible to this 

 audience. 



Of course we all know 1 now that the earth rotates, and that this 

 force of rotation is utilized to work the Gyro-Compass, but it was 

 about 1616 that Galileo first pointed out the fact, contrary to the 

 ( opernican theory, that it was the earth that rotates and the sky that 

 stood still. 



He was tried for this before the Inquisition, and, although he 

 withdrew his statement, he is said to have risen from his knees and 

 stamped on the ground, exclaiming, " Yet it does move ! " 



I shall now indicate a few of the laws of the simple gyrostat, so 

 that the descriptions of the instrument may be better understood. 



A gyrostat consists of an accurately balanced spinning wheel, 

 mounted with as little friction as possible, and in such a way that the 

 axis of the wheel may point in any direction in space. 



Mere translation in space has no action on the instrument ; 

 carrying it about, for instance, does not alter the direction of 

 the axis. 



< mi the other hand, the gyrostat is acted upon by any force that 

 tends to tilt the axis, or to give the axis a new direction in space. 



The wheel is spinning round its axis. Call the direction of this 



