298 THE POPULAR SCIENCE MONTHLY. 



ure. Tlie alcoliol in the liquid is to guard against its freezing in 

 cold weather. 



It has been stated that a single needle suspended by a thread 

 would dip more and more as one proceeded from the equator 

 toward the pole ; and that in the dry compass this is prevented 

 and the card always maintained horizontal by an adjustable 

 counterpoise on the needle : no such contrivance is needed in the 

 liquid compass ; any downward pull of the earth's magnetism is 

 at once met by such opposite pressure of the liquid on the rim of 

 the card as to neutralize it. Magnetic attraction and liquid press- 

 ure counterbalance, and the card remains horizontal. 



On the inside of the bowl is traced a fine black line — the 

 lubber' s-point, or, as it has of recent years been more appropriately 

 designated in the navy, the keel-line {L, Fig. 4). It is this line 

 toward which the point of the card indicating the ship's course is 

 always directed. The binnacle which holds the compass is 

 screwed down to the deck, so that the keel-line, as its name indi- 

 cates, is in the vertical plane through the keel of the ship, or 

 in a plane parallel to that one. This plane extends from bow 

 to stern, and divides the ship into two equal and symmetrical 

 parts. 



Now, let an observer look at the compass-card and keel-line 

 while the ship's bow swings through a portion of a circle : as each 

 point passes the keel-line, it will seem that the card itself is mov- 

 ing, but this is an illusion ; the card is still — ever pointing to the 

 magnetic pole, in obedience to the attraction that there exists for 

 the magnetism in the steel wires it carries. But it must not be 

 understood that this attraction is of a nature to pull the card off 

 its pivot : on the contrary, there is no tendency to motion of 

 translation, but merely of direction — to turn the magnets on their 

 pivot and place them parallel to the earth's lines of magnetic 

 force. 



To illustrate this, let us examine Fig. 8 : C, (7 is a steel 

 arrow free to move upon a pivot Pj from the extremities of the 

 arrow light threads t . . .t', extend and pass over revolving wheels 

 at iVand 8 ; small, equal weights Q and Q' are attached to the 

 ends of the threads. Under the strain communicated to the 

 arrow by the weights, it will, of course, lie in the straight line 

 joining the points iVand 8. 



Now, with the fingers, turn the arrow into the position C, C, 

 the threads will assume the positions t, t', and both weights will 

 be equally raised. Release the arrow suddenly, both weights will 

 descend, and alternately rise and fall as the arrow makes a series 

 of short and constantly diminishing vibrations, as shown in the 

 positions C", C and C, C, until it finally comes to rest and all 

 is still. Let us replace the arrow by a magnet, and the threads. 



