NA VIGA TION. 79 



entering from the luminiferous ether in a straight 

 line which, if continued, would pass over the 

 observer's head is bent so as to reach his eye 

 horizontally, and to make the object seem on the 

 horizon. The denser the air is, the greater is the 

 refraction ; and therefore, when, as in " lunars " 

 (56 below), extreme accuracy in the allowance for 

 refraction is required, the height of the barometer 

 and thermometer must be noted at the time of the 

 observation. The higher the barometer and the 

 lower the thermometer, the greater is the amount 

 of refraction. Books on navigation give the 

 amount of refraction for different altitudes, for 

 mean temperature and mean height of the 

 barometer, and auxiliary tables for correction 

 according to the actual heights of the baro- 

 meter and thermometer at the time of observa- 

 tion. 



47. If the earth were perfectly symmetrical 

 round its axis of rotation, like a body turned in a 

 lathe, the lines of equal latitude would be exact 

 circles in parallel planes perpendicular to the earth's 

 axis. They are not exactly so in reality, because 



