551 



Boracha were irregularly elevated. The suspended part was 

 at the northern cape five minutes long, and at the southern 

 scarcely two minutes. The first of these capes looks toward 

 the ocean, while the southern side is opposite the continent, 

 and near the island Picuita, which radiates heat during the 

 day. " When the sea is warmer in these latitudes than the 

 air, the difference of their extreme temperatures must always 

 be less on the southern, than on the northern side, whence 

 results a less negative refraction, and consequently a smaller 

 suspension." Biot, p. 238. 



During the course of my observations on the coast of Cu- 

 mana, and in other observations made on the coast of the 

 Pacific Ocean at Lima, I had carefully attended to the influ- 

 ence of the breadth of the object on the phenomenon of sus- 

 pension. I thought I had found : 1st, that in islands with a 

 convex summit, the centre of the island reposes on the hori- 

 zon, while the extremities are elevated. 2dly, that of two 

 islands of similar form, two of a parallelopiped figure for ex- 

 ample, the longest island will be elevated only toward it's 

 ends, while the shortest will be entirely suspended. The 

 fine experiments of Messrs. Biot and Mathieu on mirage 

 have perfectly elucidated the causes of all these phenome- 

 na. When a rocky island does not appear like a wall of 

 rocks perpendicularly cut at the two extremities, but is ele- 

 vated at the centre, it is only the part of the sky that re- 

 poses on it's extremities, those strata of air that appear as if 

 nearer the horizon, that can exhibit the phenomenon of 

 mirage. The aerial band, the reflected sky, will not be seen 

 under the centre of the island, in that place where it is the 

 most elevated. The same thing will happen, when of two 

 objects of similar form one has a greater length in it's lateral 

 dimensions. tf According to the theory of extraordinary re- 

 fractions near the horizon, the caustic surface rises as it's dis- 

 tance augments. The lateral extremities of an object, being 

 farther off the spectator than it's centre, will then be cut by 

 the caustic at a greater elevation. If the island have little 



