TOPS OF MASTS, 



217 



MASTS OF NEAR AND DISTANT SHIPS. 



ject approaching us from a distance, the top of it, while it 

 really remains always at the same level, appears to rise. 

 This is the explanation of the apparent rising of the clouds 

 in the western sky on a summer's day, and, in part, of their 

 increasing magnitude as they draw near. It is true that 

 clouds may, and often do, rise and descend in some degree 

 as ascending or descending currents in the air, when such 

 happen to exist, may chance to waft them. But any real 

 changes of elevation produced in this way are very small 

 and insignificant compared with the immense apparent 

 ascension of the clouds as they advance from the horizon 

 to our zenith. The whole of that, substantially, is a mere 

 illusion. 



The nature of the effect is shown clearly when we see a 



flock of birds approaching us in a long line. Those which 



are near us look far higher, when we regard their apparent 



positions as points projected against the sky, than those 



K ~ 



