EXPLANATION - OF SPECTRE SHIPS. 



217 



one b. The eye will now see through the convex lens a an 

 inverted image of the ship at S', and through the concave 

 lens b, an erect image of the ship at S", representing in a 



Fig. 35, 



general way the phenomena shown in Fig. 33. But it 

 will be asked, where are the lenses in nature to produce 

 these effects ? This question is easily answered. If we 

 take a tin tube with glass plates at each end, and fill it 

 with water, and if we cool it on the outside with ice, it 

 will act like a concave lens when the cooling effect has 

 reached the axis ; ,and, on the other hand, if we heat the 

 same tube filled with water, on the outside, it will act as 

 a convex glass. In the first case the density of the water 

 diminishes towards the centre, and in the second it in- 

 creases towards the centre. The very same effects are 

 produced in the air, only a greater tract of air is neces- 

 sary for showing the effect produced, by heating and 

 )oling it unequally. If we now remove the lenses a, b, 

 id hold a heated iron horizontally above the water in 

 trough ABC, the heat will gradually descend, 

 tpanding or rendering rarer the upper portions of the 

 luid. If, when the heat has reached within a little of the 

 >ttom, we look through the trough at the ship S in the 

 lirection E S', we shall see an inverted image at S', and 

 erect one at S", and if we hide from the eye at E all 



