152 LETTEBS ON NATUEAL MAGIC. 



at Ramsgate, an eye at a greater distance from 

 the castle, and in the path of the rays, would 

 have seen the image inverted. This will be 

 better understood from the preceding diagram, 

 which represents the actual progress of the rays, 

 from a ship SP, concealed from the observer at 

 E by the convexity of the earth P Q, E. A ray 

 proceeding from the keel of the ship P is refracted 

 into the curve line P c x c E, and a ray proceed- 

 ing from the top-mast S, is refracted in the 

 direction S d x d E, the two rays crossing at x 9 

 and proceeding to the eye E with the ray from 

 the keel P uppermost; hence the ship must 

 appear inverted as at s p. Now if the eye E of 

 the observer had been placed nearer the ship as 

 at x, before the rays crossed, as was the case at 

 Ramsgate, it would have seen an erect image of 

 the ship raised a little above the real ship SP. 

 Rays S m, S n^ proceeding higher up in the air, 

 are refracted in the directions S m m E, S n n E, 

 but do not cross before they reach the eye, and 

 therefore they afford the erect image of the ship 

 shown at s' p'. 



The aerial troopers seen at Souterfell were pro- 

 duced by the very same process as the spectre of 

 Dover Castle, having been brought by unequal 

 refraction from one side of the hill to the other. 

 It is not our business to discover how a troop of 

 soldiers came to be performing their evolutions 

 on the other side of Souterfell ; but if there was 

 then no road along which they could be march- 

 ing, it is highly probable that they were troops 

 exercising among the hills in secret, previously 

 to the breaking out of the rebellion in 1745. 



The image of the Genevese barque which was 



