WORKING WONDERS WITH A TOP 



Commenting on this, Mr. Sperry says : "What 

 better compass could one wish than that suggested 

 in the last clause of the above quotation from the 

 original writings of Foucault?" And a moment's 

 reflection shows the force of this suggestion. The 

 axis of a gyroscope pointed at the pole star, that is to 

 say due north, will obviously constitute a compass of 

 a novel and important kind. 



Such an apparatus, however, without further modi- 

 fication, would by no means serve as a practical ship's 

 compass; for in the first place it would require to be 

 sighted or aimed before it would be of any service, 

 and secondly, it would have no power to readjust it- 

 self should it by any chance be thrust out of position. 

 A practical compass must obviously be one that can 

 find the north for itself and return to position if de- 

 flected. In order that the gyroscope shall meet this 

 test, it is necessary to restrict its freedom of action. 

 Suppose, for example, that the rotating wheel were 

 suspended in such a way that its secondary axis must 

 maintain a horizontal position, though free to oscil- 

 late and point in any direction in the horizontal plane. 

 It appears, rather paradoxically, that such restriction 

 is equivalent to putting the gyroscope in harness and 

 making it serviceable. 



How this comes about will be understood if we re- 

 flect that any object here at the earth's surface that 

 maintains a horizontal position in any direction other 

 than that of the meridian is in* reality constantly 

 changing its position in absolute space. Lay a pen- 

 cil on the table before you, placing it in the east and 

 west line. Obviously the position of that pencil in 



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