102 POPULAR LECTURES AND ADDRESSES. 



57. The details of the method of lunars, the 

 practical mastery of which used to be the great 

 test of a good navigator before the time of 

 chronometers, are beyond the scope of the present 

 lecture. I must limit myself to telling you that 

 from rough observations of the altitudes of the 

 two bodies, moon, and sun or planet or star, and 

 observations of the barometer and thermometer, 

 the effects of atmospheric refraction in altering 

 the apparent distance between the two bodies 

 must be calculated. By an approximate know- 

 ledge of the ship's position, the difference between 

 the observed distance and that which would have 

 been observed if the place of observation had been 

 the earth's centre, must be determined. 



The application of these corrections for refrac- 

 tion and parallax, so as to find, from the observed 

 distance, the actual angle between the line going 

 from the earth's centre to the moon's centre, 

 and the line from the earth's centre to the other 

 body, is what is technically called " clearing the 

 distance." 



58. The books on navigation used at sea 



