r>26 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



objects only can be seen that are not much above the horizon at 

 greatest 12 to 20. 



The variation compass is sometimes placed on a platform above 

 the dome over the after-cabin stairs. 



Travellers in their geographical explorations of the interior of 

 continents, and geologists who wish to know the directions of moun- 

 tain chains, or of the other surface 

 features, employ the compass like 

 sailors. Only as the instrument is 

 more easily set up in a fixed position 

 there is no need for so complex a 

 method of suspension. It is enough 

 to have a tripod stand, to which the 

 compass is fixed by a ball and socket 

 joint, and a spirit level to secure 

 the horizontality of the reading 

 circle. A small telescope, with cross 

 wires, which moves parallel to the 

 north and south line on the compass 

 in a vertical plane, enables the ob- 

 server to look in the direction of the 

 line whose orientation has to be 

 measured. The more simple com- 

 passes have a sight- vane with pinules 

 instead of a telescope. 



Since the compass enables us, 

 when the magnetic variation of a 

 place is known, to find rapidly the 

 angle which any line makes with 

 the meridian that is its orientation 

 it is clear that if we have deter- 

 mined in this way the azimuthal angles of a series of horizontal 

 lines, say those of the sides of a polygon, it is only necessary to take 

 the difference between these angles in order to obtain the angles 

 which these lines make with each other ; and more than this, if we 

 only require to know the angles of the polygon, they may be obtained 

 in the same way, without our being obliged to know the declination 

 of the place. It is sufficient that during the operation the direction 



FIG. 340. Surveying compass. 



