552 



width, the difference will be imperceptible, and the whole 

 island will appear almost equally suspended in the air. But 

 if we observe an island rather long, the borders of which 

 belong to distances much more considerable than it's centre, 

 the difference of height of the caustic at these various dis- 

 tances may become more apparent, and the lateral extremi- 

 ties of the island will then alone be suspended. If the differ- 

 ences of temperature gradually augment, and the point of 

 contact of the limiting trajectory approach nearer the spec- 

 tator, or, what is the same thing, if the spectator lower his 

 position, the limiting trajectory may rise above the summit 

 of the island, which will then appear entirely suspended in 

 the air." Biot, p. 212. Thus I find noted in my journal, on 

 the 7th of September, on the coast near the mouth of the 

 Rio Manzanares, at the foot of the Fuerte de la Bocca, therm. 

 19° R. ; hygr. 43*2° ; height of the eye four feet three 

 inches ; at 19 h in the morning, on the quadrant I had 

 conveyed to the seaside, C. 95° 91*3. The body of the 

 island rests on the horizon of the sea. The extremities alone 

 are elevated. Water of the sea near the shore, 20*2° R. 

 At 21h, therm. 20*2<> R, • hygr. 42'8<>. C. 95® 91'8, but the 

 whole of the island, the centre as well as the extremities, 

 suspended. Water of the sea covering a shore heated by the 

 Sun, 21'8 0 . The sand 26<> R. 



What has just been said, relative to the influence of the 

 length and form of objects on the phenomena of suspension 

 appears to me, to lead to the explanation of a curious pas- 

 sage of Theophrastus, in his book on the Prognostics of 

 Wind. " When the capes (or saliant parts of the coast) seem 

 suspended in the air ; or when, instead of one island, we think 

 we perceive several, this phenomenon indicates a change of the 

 south wind. When the land appears to you black (detaching 

 itself dark) you will have a north wind $ if it appear white 

 (detaching itself light) expect a south wind." Theophr. de 

 Hgnis ventorum, 422, B. edit. Heinsii. Furlanus translates it 

 thus : si promontoria sublimia, insulaeve si ex una plures ap- 



