68 POPULAR LECTURES AND ADDRESSES. 



This method is much used by naval officers to 

 measure the distance at any moment from the 

 admiral's ship, or some other ship, when sailing in 

 squadron, as an aid to keeping in station. 



II. ASTRONOMICAL NAVIGATION. 



42. Before attempting to explain Astronomical 

 Navigation, I must tell you something of the earth 

 as a whole. 



When you look at the hills you see that the 

 earth is not exactly globular ; but if I could show 

 you an exact model of the size of this large globe be- 

 fore you (of two feet diameter), with every mountain 

 chain, and hill, and valley, and tree, and building 

 constructed exactly to scale, and with the whole sea 

 solidified in the form ruffled by waves, precisely as 

 it is at any instant, you could not perceive without 

 minute and careful examination, that it was any- 

 thing different from an exact sphere. The 

 Himalayas and Andes would be barely perceptible 

 roughnesses, the highest of them being about I 60 

 of an inch. The greatest buildings of the world, 



