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THE AMERICAN MUSEUM JOURNAL 



weather, the tops of the four highest towers 

 of Dover Castle may be seen. The remainder 

 of the edifice is concealed by a hill, which is 

 about twelve miles from Ramsgate. On the 

 6th of August, 1866, at seven in the evening, 

 the four towers were not only to be seen, but 

 also the entire castle from roof to base. 



(16) — Upon the shores of the Orinoco, 

 Humboldt and Bonpland observed that the 

 hillocks of San Juan and Ortiz and the Galera 

 IVIountains sixteen miles distant, seemed to be 

 suspended in the air; the palm trees appeared 

 to have no hold on the ground. In the midst 

 of the plain of Caracas, these men saw at a 

 distance of a mile and a half a herd of oxen 

 apparently in the air. Humljoldt also no- 

 ticed a herd of wild cattle, part of which 

 seemed to be above the surface of the ground, 

 while the remainder were standing upon the 

 soil. 



(17) — Borchgrevink, the first man to 

 land on the Antarctic continent, observed in 

 Victoria Land in 1899 — -when he was leader 

 of an expedition to that region — both during 

 the time the sun was low in its descent and 

 when it rose again, a strong mirage effect 

 toward the west, showing images of icebergs 

 far below the horizon, and Antarctic scenery, 

 visible to them only through this phenome- 

 non. This strong mirage remained after the 

 sun's return late in the summer, and the open- 

 ing of the ice was prophesied thus long before 

 the ice fields near Victoria Land ]:»roke up. 

 This Antarctic party watched the northwest- 

 ern sky with interest, for they could see the 

 far-away broken ice fields with their dark 

 channels and towering bridges, and on sev- 

 eral occasions the men became enthusiastic, 

 thinking that they had discovered the masts 

 of the "Southern Cross," their ship, in the 

 mirage. 



(18) — From their southernmost point on 

 the Ross Barrier, December, 1902, Captain 

 Scott and his companions saw long snow capes 

 running out beyond Mount Longstaff and 

 meeting the level horizon of the barrier, 

 while farther still the mirage threw up small 

 white patches against a pale sky which were 

 indicative of still more distant capes and 

 mountains. The direction of the extreme 

 land thrown up in this manner was south 

 17° E., and hence they could say with cer- 

 tainty that the coast line, after passing Mount 

 Longstaff, continues in that direction for at 

 least a degree of latitude, that is, approxi- 

 mately seventy miles. From this they felt 



sure that the high mountainous coast line 

 does not turn to the east before reaching the 

 S4th parallel. Amundsen, the Norwegian 

 explorer, on his dash to the South Pole, 

 1911, records that the range did not turn to 

 the east before it crossed the 85th parallel. 

 A mirage thus assisted Scott and his compan- 

 ions, who knew well the apjjearance of a 

 snow-capped country, in detecting objects 

 beyond their normal range of vision. 



When we coordinate these descriptive 

 records and the results of the two experi- 

 ments previously noted, we find that the 

 interest centers about the number, kind, 

 and association of the images produced 

 by light as it passes from distant objects 

 through or across layers of air (or other 

 media) having different densities. 



The upper portion of the tank experi- 

 ment develops one kind of mirage, the 

 lower portion another. Between posi- 

 tion 2 and 4- [lower figure, page 514] the 

 alcohol and water media produce three 

 images in a vertical plane, the middle 

 one being inverted. In the second ex- 

 periment and in observations (1), (2), 

 (3), and (4), only the inverted and the 

 superimposed erect images appear. The 

 lower image fails to develop. 



Optically alcohol is denser than water, 

 hence in the upper part of the tank the 

 optically denser medium is above, the 

 rarer below. To produce similar illu- 

 sions in the field the same relation of 

 density must exist. This is not the 

 normal condition of the atmosphere, but 

 since there are often temperature varia- 

 tions, it may develop that air layers, in 

 contact w^ith the heated ground or warm 

 water, may be very much heated for a 

 short distance up, producing an unstable 

 condition in the atmosphere. This con- 

 dition does not extend very far laterally 

 and much less verticall,y, and conse- 

 quently in no wise affects the general law 

 of the decrease in density in the atmos- 

 phere upward. This variation in den- 



