220 LETTERS ON NATURAL MAGIC. 



The aerial troopers seen at Souterfell were produced 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 dis- 

 cover 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 marching, it 

 is highly probable that they were troops exercising among 

 the hills in secret previous to the breaking out of the 

 Eebellion in 1745. 



The image of the Genevese bark which was seen sailing 

 at a distance from the real one, arose from the same 

 cause as the images of ships in the air, with this difference 

 only, that in this case the strata of equal density were 

 vertical or perpendicular to the water, whereas in the 

 former cases they were horizontal or parallel to the 

 water. The state of the air which produced the lateral 

 image may be produced by a headland or island, or even 

 rocks, near the surface, and covered with water. These 

 headlands, islands, or sunken rocks being powerfully 

 heated by the sun in the daytime, will heat the air 

 immediately above them, while the adjacent air over the 

 sea will retain its former coolness and density. Hence 

 there will necessarily arise a gradation of density varying 

 in the same horizontal direction, or where the lines of 

 equal density are vertical. If we suppose the very same 

 state of the air to exist in a horizontal plane which exists 

 in a vertical plane, in Fig. 36, then the same images 

 would be seen in a horizontal line, viz., an inverted one at 

 s p, and an erect one at s p. In the case of the Genevese 

 bark the rays had not crossed before they reached the eye, 

 and therefore the image was an erect one. Had the real 

 Genevese bark been concealed by some promontory or 

 other cause from the observation of MM. Jurine and 

 Soret, they might have attached a supernatural character 



