CHAP. I.] 



TEE COMPASS. . 



525 



from it essentially in having the telescope replaced by a concentric 

 sight-vane with pinnies PP' at opposite extremities of a diameter. 

 The case containing the magnetic needle with divided edge is sus- 

 pended on gimbals. Two wires crossing at right angles are stretched 

 over the side of the case containing the needle, and one of them gives 

 the direction of the slits in the pinnies, and consequently that of the 

 plane of vision. One of the pinules carries a mirror at an angle of 

 45, in which the observer sees the arc of the card and the correspond- 

 ing divisions at the same time that he sees the star through the slit of 



Bffllul ^^ 



FIG. 338 Variation compass. 



FIG. 339. Portable declination compass. 



the pinule and a part of the mirror where the quicksilver has been 

 removed. 



While one observer sees the star, planet, moon, sun, or a terrestrial 

 object, and reads by the mirror the division which shows the angle that 

 the magnetic needle makes with the vertical plane of the object 

 seen, another observer makes a second reading by means of a thread, 

 which is stretched at right angles to the direction of the pinules ; this 

 second reading serves to control the first. With this instrument those 



necessary correction may be made ; or another means may be employed, that of 

 placing, in convenient places, bars of soft iron or magnets calculated to destroy the 

 deviation. Unfortunately, it happens, that during the voyage, the magnetism of the 

 ship changes in direction and intensity, and then the risk is so much the greater as 

 there is believed to be none. 



We have here a most interesting problem, of which the solution is still being studied 



