174 TEE CAUSE OF MIRAGE. 



electro-magnet, and as instantly loses all traces of magnetism when the 

 current is broken. It is supposed that in soft iron the atoms swing easily 

 and hence the instant conversion and reversion that is observed. In steely 

 which is hard iron, because carbonized aad hence compound, the atoms 

 swing only gradually and that by coercion; hence though steel takes on 

 the magnetic influence slowly, yet the magnetism persists after the current 

 ceases to flow, and the steel bar or needle, as may be, remains a permanent 

 magnet. 



A ray of polarized light passing through a transparent medium, is de- 

 flected to the right or left of its plane of polarization according as the trans- 

 parent medium is under the influence of the north or south pole of a mag- 

 net. A polarized heat ray passing through a crystal of rock salt is deflected 

 from its plane as by the action of the poles of a magnet. 



These cases, we think, justify the inference that light will be similarly 

 acted upon by any transparent electrified medium. Magnetism is an effect 

 of electricity, and certainly it is entirely logical to infer that if electricity 

 causes this deviation of light mediately by the indirect action of magnet- 

 ism, then it can -and does cause it immediately by direct action. Air is a 

 transparent, or rather translucent medium. That in calm air or nearly so^ 

 under a high barometer, the atoms of oxygen and nitrogen have greatest 

 positive electric tension ; and greatest negative electric tension under a low 

 barometer, are facts well established by observation. Deviation of the rays 

 of light from a straight line, causes the phenomenon of mirage; since ex- 

 periment proves that electric media cause such deviation, hence it must be 

 inferred that the abnormal electric tension of the atmosphere — character- 

 istic of the centres both of high and low barometers — causes the deviation of 

 light that produces the phenomenon of mirage, that is, causing objects to 

 appear above the horizon when they are actually below it. 



At other times the atmosphere acts like a mirror, giving rise to two 

 species of phenomena: (1) where objects below the atmosphere and beyond 

 the stratum of atmosphere reflecting them are represented as inverted; and 

 (2) where the objects on the same side as the observer are reflected back as 

 if the atmosphere were a mirror. 



■ Fremont, in his journals of his expeditions in the Rocky Mountains^ 

 gives an example of this reflection. One day, while in the Utah Basin, they 

 observed a party approaching some miles distant, moving as though they 

 designed to cut off Fremont and his party. The latter halted, and instantly 

 the phantom party halted also. Fremont now detailed one of his party to 

 interview the opposite party, to ascertain what they wanted. Instantly 

 one of the opposite party also came forward to meet the envoy. Then the 

 true state of the case was first perceived, that is, that the suspicious party 

 with such sinister movements was no other than their own images reflected 

 as in a mirror. 



On the 29th of March, 1874, I saw a similar phenomenon at Boulder 

 City, Colorado. The day before a heavy snow storm had prevailed, and 



