328 POPULAR LECTURES AND ADDRESSES. 



the two sides of a vertical plane through its centre. 

 The sole-plate rests on three feet, one of which, 

 under the centre of gravity of the deflector, rests 

 in the conical hollow in the centre of the glass. 

 It is caused to press with a small part of its whole 

 weight on the other two feet by a brass spring 

 attached to the bottom of the sole-plate on the 

 other side of the centre from these two feet, and 

 pressing downwards on the glass. A brass pointer 

 attached to the sole-plate marks the magnetic axis 

 of the deflector. It projects from the centre, on 

 the side of which is the pair of true north poles. 

 Thus, if the deflector be properly placed on the 

 glass of the compass-bowl, with the pointer over 

 the north point of the card, it produces no deflec- 

 tion, but augments the directive force on the needle. 

 To make an observation, the deflector is turned 

 round in either direction, and the north point of 

 the card is seen to follow the pointer. The power 

 of the deflector is adjusted by the screw, so that, 

 when the pointer is over the east or west point of 

 the card, the card rests balanced at some stated 

 degree of deflection, which for the regular observa- 

 tion on board ship is chosen at 85. A scale, 

 measuring changes of distance between the 

 effective poles of the deflector, is then read and 

 recorded. For adjusting compass by aid of the 

 deflector, the magnets are so placed that the 

 deflector reading, found in the manner just 

 described, shall be the same for the four cardinal 



