Mar. 1 6, 1888.] 



SCIENTIFIC NEAAAS. 



255 



workman who is able and willing to sacrifice his son's 

 earnings by keeping him at school till he is fourteen or 

 fifteen ; while, on the other hand, the teaching must be 

 so good that the wealthy employer will find it worth 

 while to send his son to the local school. The son of 

 the employer and of the workman will thus be found 

 side by side at the same desk, and it is not likely that the 

 mutual contact will injure the one or the other. Just as 

 efficient teaching brings good grants, so it contributes 

 powerfully towards its own cost, and in this respect the 

 financial difficulty of giving to artisans the benefit of the 

 best instruction will not be serious. Nevertheless, any 

 deficiency between the income and expenditure would 

 need to be met by public sources, the most appropriate 

 being the local rates. 



[To be continued.) 



— <T>i^'^5^^ — 



MIRAGE. 



ONE of the most curious of meteorological phenomena 

 is that known by the French as mirage, by the 

 Arabs as sirab, by the Sicilians asfata morgana, and by 

 the people of the German Baltic coasts as kintniung. Its 

 English name, if it had one, would probably be 

 " kimming," though one form of it is known by our 

 sailors as " looming." 



The essential feature of this phenomenon is that objects 

 whi'ch are below the spectator's horizon, and conse- 

 quently invisible in the ordinary state of the atmosphere, 

 are seen sometimes inverted, sometimes upright, and 

 either suspended in the air or as if reflected from a 

 mirror laid on the surface of the earth. This illusion is 

 most common in the Soudan, in the desert parts of Egypt, 

 in the Sahara, Arabia, and Persia. The traveller sees at 

 some little distance before him a tranquil lake, in which 

 are reflected palm-trees, houses, and other prominent 

 objects. He pushes on, in the expectation of finding 

 water, the great desideratum in those regions, but finds 

 the vision recede before him, or perhaps vanish alto- 

 gether. 



Similar illusions are witnessed at sea, always, how- 

 ever, in calm weather, and in curious contrast to the 

 desert cases for the most part in high latitudes. Ships 

 engaged in the whale-fishery have sometimes been seen 

 and even recognised by means of the mirage when at 

 very considerable distances. Perhaps the best-authenti- 

 cated case of this kind is that recorded by Capt. Scoresby. 

 He saw and identified his father's ship when it must 

 have been more than thirty miles off, though he was not 

 at the time aware that it was cruising in that region of 

 the Arctic seas. It appeared to him as if inverted and 

 suspended in the air. 



One of the most curious forms of the mirage is that 

 sometimes witnessed in the straits of Messina, espe- 

 cially near the town of Reggio. A strange assemblage of 

 objects, some stationary and others moving, are described 

 as having been seen, either in the air or on the surface 

 of the sea. Minasi gives a rather sensational account of 

 this phenomenon which has been copied as literal truth 

 into a variety of works — even into treatises on optics. 

 He writes : — " A spectator, on an eminence in the city of 

 Reggio, with his back to the sun and his face to the 

 sea, and when the rising sun shines from that point 

 whence the incident raj's form an angle of about 45° on 

 the sea of Reggio, sees upon the water numberless 

 series of pilasters, arches, castles, regular coliimns, lofty 



towers, superb palaces, with balconifs and windows, 

 villages and trees, plains with herds and flocks, armies 

 of men on foot and on horseback, all passing in rapid 

 succession along the surface of the sea." 



This phantasmagoria is ascribed to the agency of the 

 Fairy Queen, Morgana la Fata, of popular mythology. 

 Now, here is precisely the difficulty. Such accounts are 

 almost always so overlaid with imaginary embellishments 

 that it is difficult to come at the actual facts. Of these cases 

 at Reggio Captain Smyth in his work on Sicilyobserves : — 

 '' I never met with a Sicilian who had actually seen any- 

 thing more than the loom or mirage, consequent on a 

 peculiar state of the atmosphere ; but which, I must say 

 I have here observed many times to be unusually, 

 strong." 



We have met with accounts of various phenomena 

 said to have been witnessed in various parts of Britain 

 and Ireland, which have been referred to the " second 

 sight," or to other extra-natural agencies, but which have 

 probably their substratum of fact in this same optical 

 phenomenon. In the East the Arabs consider the mirage 

 to be the work of sorcerers or of Djinns, who wish to 

 entice a caravan out of the right track in the fallacious 

 hope of meeting earlier with water and shelter. 



But leaving superstitions on one side, there are 

 good reasons why our accounts of the mirage should 

 be exaggerated, if not distorted. The traveller in the 

 desert, tormented with real thirst, as distinguished from 

 the mere acquired craving for liquid so common in Eng- 

 land, will naturally see a luxuriant landscape, where 

 there is even the slightest indication of water and of 

 shade. If we turn to the cause of the mirage we shall 

 soon be able to distinguish in these narratives the truth 

 from the marvels which have been superadded. 



The whole matter, as explained by Monge, is a case of 

 refraction. It results from the unequal density of dif- 

 ferent layers of the air, when they are rarefied by 

 contact with the heated soil. The lowest stratum of the 

 air is then the least dense. Hence a ray of light reflected 

 from any elevated object passes through layers of air, 

 which are gradually less and less refractive, and is again 

 reflected from the lower surface of such a stratum, thus 

 presenting to the eye an inverted image of the object, as 

 if reflected in a sheet of water placed somewhere be- 

 tween its real position and the eye of the spectator. 



As a matter of course the mirage, to the view of a 

 cool judicious observer, neither tormented with thirst 

 nor influenced by superstition can create nothing. It 

 must merely present the images of objects which are 

 really existent, although at a much greater distance than 

 it would be supposed. 



The cases when ships are seen suspended in the air 

 are due to the same law of refraction. In the Arctic 

 seas the layer of air in immediate contact with the water 

 will be the coldest, and consequently the densest and 

 most strongly refractive. Hence the rays of light re- 

 flected from the vessel are bent upwards. 



Wollaston devised a method of exhibiting the mirage 

 artificially. He looked along the side of a red-hot poker, 

 at an object ten or twelve feet off. And about 3-8ths of 

 an inch from the end of the poker an inverted image of 

 the object was seen. 



It need scarcely be said that the mirage can occur 

 only in calm weather. Wind prevents the formation of 

 denser or rarer strata of the air by mixing all up to- 

 gether, and consequently gives no scope for the effects of 

 refraction above mentioned. 



