EXPLANATION OF PHENOMENA. 153 



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 perpen- 

 dicular 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 neces- 

 sarily 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 horizon- 

 tal line, viz., an inverted one at s p, and an erect 

 one at s' p'. In the case of the Genevese barque, 

 the rays had not crossed before they reached the 

 eye, and therefore the image was an erect one. 

 Had the real Genevese barque been concealed 

 by some promontory or other cause from the 

 observation of Messrs. Jurine and Soret, they 

 might have attached a supernatural character to 

 the spectral image, especially if they had seen it 

 gradually decay, and finally disappear on the still 

 and unbroken surface of the lake. No similar 

 fact had been previously observed, and there were 

 no circumstances in the case to have excited the 

 suspicion that it was the spectre of a real vessel 

 produced by unequal refraction. 



