EXPLANATION OF SPECTRE SHIPS. 



151 



above will give out its heat to the cooler air 

 immediately below it, so that the air from the 

 surface of the sea, to a considerable height 

 upwards, will gradually diminish in density, and 

 therefore must produce the very phenomena we 

 have described. 



The phenomenon of Dover Castle seen on the 

 Ramsgate side of the hill, was produced by the 

 air being more dense near the ground and above 

 the sea than at greater heights, and hence the 

 rays proceeding from the castle reached the eye 

 in curve lines, and the cause of its occupying its 

 natural position on the hill, and not being seen 

 in the air, was that the top of the hill itself, in 

 consequence of being so near the castle, suffered 

 the same change from the varying density of the 

 air, and therefore the castle and the hill were 

 equally elevated and retained their relative posi- 

 tions. The reason why the image of the castle 

 Fig. 36. 



and hill appeared erect was, that the rays from 

 the top and bottom of the castle had not crossed 

 before they reached Ramsgate ; but as they met 



